Some of you may have already seen a few of my big bore build threads on other forums but thought I share a few here for those that are interested. 
My mission is to build a lightweight short .30cal big bore PCP from scratch suitable for taking deer and pigs with cast projectiles. 
I have 3 different barrels to test with a range of twist rates and bore diameters for the range of .308 cast projectiles which are in themselves tunable in weight and balance with an adjustable depth hollow point.Ive got my money on the 1-14" twist .307.5 match barrel for the 105gr cast projectiles but also have a 1-18" twist for 80gr -120gr projectiles and a 1-10.5" twist shallow cut microgroove for up to 160gr projectiles. This is the bullet I have chosen, it is a 115gr Lyman bullet 
The mould has been modified into a hollow point that now casts a 95-110gr HP pure lead bullet depending on hollow point depths. 
My goal is to get about 170-180ftlbs from this rifle so although making a bullet for a barrel before the rifle is even built may seem a little backwards but it was a big part of the R&D process for this project and I needed to be very sure there was a bullet that would suit my needs and will be stable at a lower than intended velocity range in one or more of my barrels. I pushed a sized one through the microgroove bore to get a feel of the bore friction and check out the rifling groove cuts in the bullet.. the weight and balance can be fine tuned by altering the hollow point depth and diameter at a later date if needed. 
For the action I cut a piece of 1-1/4" 6061 T6-51 alloy to length and skimmed one side down on the mill to 1" to form the start of the receiver. 
Then I marked the barrel/bolt centre, fitted up my 4 jaw chuck and drilled the pilot hole with a long series 8mm drill bit.. 
Followed by the 1/2" barrel clearance size drill and a 10mm bore for the bolt.I then cut the loading port for the feed tray and the 45 degree chamfers to pretty it up.. 
A little more milling and the basic shaping is done.. It will get a raius to mate up with the cylinder, transfer port and then the bolt and cocking pin cut outs on the underside.. 
A few more components to the list.. the rear plug is yet to be bored out and threaded for an external power adjuster/spring guide and the bolt still needs the probe, O ring groove and handle to be made.The 1-14" twist match barrel has been pre fitted and is ready for the transfer port to be drilled. I have decided to use a modified German made Gecardo trigger because Im too lazy to make one from scratch.. 
Progress has been a little slow this week as I have been a little pre-occupied but I spent a little time on the lathe tonight and made the bolt handle.Because of the heavier hammer springs needed on big bore airguns, there is a trend to have a separate cocking handle directly into the hammer and protruding from the cylinder but I'm not a big fan of that "double action" system. 
This is normally done to lessen the cocking pressure but I have decided to have the hammer cocking of the bolt so it had to be built tough and of a big enough diameter to take extra load After shaping and trimming to length I heated it red hot and put a nice bend in it. It is drilled into the bolts shaft and held tight in place by a recessed 4mm cap screw. 
Next up the high flow valve.. I cut a length of 6061 T6 bar a little longer than I need for the valve but Ill use the other half on the front plug/fill valve.After turning to the inside diameter of the air cylinder I drilled a 5mm pilot hole right through and then parted it off a little over the length it will end up and bored the various port, valve shank and seat internal diameters. 
Once all the internal bores were cut I measured up and drilled the transfer port and then milled a counter sink for a transfer port tube. 
The valve was then slipped into the air cylinder and located it with the transfer tube. Once locked in place I drilled the holes three valve retaining cap screws and countersunk them with a 3/8"mill cutter so the heads of the cap screws are in shear load not the threaded shanks. This "pinning" system is far stronger and safer that threaded end plugs that can strip out and thin the cylinder walls in the threaded sections. 
Back in the lathe for a polish and to cut the O ring grooves 
Then a hole and 1/8" BSP tapered thread to fit a downward facing manometer. 
With a final polish and clean it is ready for assembly. The gauge will get a brass extension to drop it down flush with the walnut stock so I haven't wound it tight into the tapered threads 
A little more progress in the last few days...I decided to use a fill probe rather than a foster fitting to keep the cylinder shorter and tidy so started by turning a fill probe out of a piece of 12mm brass hex rod and milling a blind 8mm hole into another chunk of alloy bar. I wanted to leave it "blind" so the fill port was only visible from the underside of the cylinder and lessen the risk of introducing debris in to the air cylinder and valve etc. 
Once the plug was turned to a nice slip fit into the cylinder and fitted with an O ring I centre drilled the 3/32" air inlet port with a long series drill bit into the 8mm fill probe port and cut a 3mm thread for a countersink head cap screw to create the one way fill valve. 
I assembled the simple one way valve design consisting of a 3mm countersunk cap screw with the thread filed flat on one side snugged down on a 005 O ring and tested/adjusted its flow rate on the dive bottle.. I adjusted it to a restriction pressure of 500psi on a fully open dive bottle valve (3200psi) which will be enough to let air in slowly but completely seal and not let air back out when the probe is removed. Works a charm.. ;D 
The front plug is held in place by three countersunk 12.9 grade cap screws like the valve body again with the heads in shear load not the threads. 
A small mod was needed after initial testing as it was hydraulicing the air in front of the probe which prevented me from pushing it all the way home.. a 1mm vent port drilled into the top side of the fill probe hole sorted that out and allowed the probe to seat all the way home. 
So with both shiny new parts fitted up to the air cylinder I filled it with air and submerged it in a bath of water to check for any leaks then left it over night to check for any leak down.. Sunrise came and the cylinder hasn't dropped any pressure at all. 
With the air cylinder sealed up and the valve working I moved onto fitting the receiver to the cylinder.The transfer port was used to centralise the action on the cylinder which consists of a steel transfer tube compressed between the two parts in blind counter-bores sealed with external O rings. 
Once the transfer port was mated up I drilled the action for four flush fitting 4mm cap screws to lock it all down in place 
After some careful measuring of the hammer travel and total spring compression range I then cut a slot for a cocking pin and mated it up with a second slot in the receiver for the opposing cocking pin/bolt lock up. I have used a much bigger cap screw in the bolt and recessed the head a little as it is subjected to a lot of rearward pressure on discharge, I will also be adding a second smaller "safety screw" on the bolt in case the main locking screw ever failed and tried to exit the action rearward. Its not as much as a safety hazard in a bullpup as it would exit over top of your shoulder and probably leave you with a ringing ear but in a conventional rifle set up the bolt is right in front of your nose :shock: 
The action works smoothly but it does takes quite a bit of effort to compress the hammer spring with the added side loadings of the cocking pins etc so regretfully I think I may have to cut a second slot in the side for a stand alone direct cocking handle. On a positive note: the proportions look pretty sweet in its walnut bullpup stock. The .308 match barrel is currently cut to 23 inches, this cast bullet barrel will be a deer, pig and goat slayer so noise is no issue and will probably stay unsuppressed.. I may make a nice break for the muzzle though. 
Ive had a few emails from guys asking where I got this bullpup stock from, it is one I made a couple of years back for a .25cal Marauder but it is a bit of a cupboard queen at the moment so I have decided to share the stock and the forward scope riser mounts/trigger group with my new .30cal build. So for those of you that missed that build thread here's a brief look at the making the stock.. (sorry to all those that have seen it before) The stock started as a large 6 foot long 2.5" thick slab of 120+ year old black walnut.. I spent about an hour moving the template around the slab trying to envision the finished stock... Im very happy with the chosen piece but wont really see the true grain until I start the shaping. I should have taken a pic of the vast array of router guides and jigging I had set up to get the inletting right, It looked awkward to put it best.. This is my first stock Ive made by hand that is not to be a painted finish so Ive been sooooo careful not to slip with any power tools..that said there's a lot of work to do on it yet so if you see a sudden shift from the idea of polished walnut to a nice olive drab paint job, you can assume Ive muffed it up and had to bog something.... 
I spent about an hour behind the router tonight snorting sawdust and reminiscing my college wood working class...I made a good start on the inletting. Nice snug fit too. 
The scary inletting job is all done, it was a little more complex than I first thought so I'm very relieved to have that stage finished..Another couple of hours behind the router and its starting to take shape..I had to shift the lateral router line down about 10mm due to the thin wood in the scope mount recesses. I wanted to be well clear so not to weaken it or in case they "broke through". It hasn't really changed the look to the original design though, they still look good.I really wanted to stray away from the "chunky" looking bullpup designs commonly seen and endeavour to produce a thinner, slick, light weightand more ergonomic stock. 
I finished cutting the basic outline now and have started roughing out the contours.. Its starting to show some really nice grain now.. 
I've got it all shaped out now which only leaves the sanding part... lots and lots of sanding!I'm really loving the lines of this stock, the grain follows the curves just as I hoped it would whenI laid out the template. I have tried to keep the Eastern European stile lines which I'm quite fond of.Probably not every bodies cup of tea.. but then again nor are bullpups 
A total of 20 hours work and the stock is finished just applied the final coat of oil. with the recoil pad fitted the stock weighs 980 grams. 
...and here's the forward trigger group and scope mount risers I will also be re-using: 
A little more progress in the last few days.. Regretfully I cut another slot in the side of the cylinder to add a stand alone cocking handle, not really my preference but it is much better in terms of smooth operation. it cocks with very little effort now and the cocking handle has added weight to the hammer which may eliminate the need for an internal tuning weight. I have left it quite long but will cut it down a little once I shoot a few strings. I also spent a little time polishing the machining marks out of the various parts, eventually after all testing is done the external alloy parts will be bead blasted and anodized flat black. 
I cut the bolt probe to length and fitted two 007 O rings (cool size huh) :mrgreen: to it, it seals just inside the breach against the receiver itself ant the other sits just outside the transfer port in the barrel. I am a fan of at least two O rings in this position as there's nothing worse than a breech leak on a bullpup.. loud and uncomfortable is the best case scenario and a permanently flared nostril and an eyelid turned inside out is the beginning of something quite dangerous.. 
I have shortened the bolts stroke to just allow a bullet in the loading tray by a second safety cap screw in the bolts shaft and added a sprung ball bearing with a detent in the bolt shank to positively lock it up in the closed position. It all runs very smooth now. 
The 26".308 match barrel started its life on a .308 win target rifle and after the .308 Win chamber section was removed it showed signs of a section of throat erosion which is common and it had a an inch or so or wear at the muzzle end too probably from excessive cleaning from that end.I have trimmed both ends back to remove the bad sections and have ended up with 21" of good strong shiny rifling to work with.. I have added a muzzle break to keep the aesthetics looking right because it ended up an inch and a half shorter than I'd hoped but"accuracy first" 
Thanks to my gunsmith friend for the loan of a piloted .308 tapered throat ream I have the cast bullets slipping in with perfect resistance and a nice gradual taper from the lands to the groove without any angled shoulder to gather lead.This match barrel is tight, it slugs at .3072" so I will be sizing my bullets to .308" not .309" like I had intended for a .308 bore. Ill be very keen to see these on paper.. 
Nearly finished.. I still have to modify the scope riser mounts for this barrel and set up the trigger linkage..It is VERY light and will be a pleasure to carry, it is 30" over all length. 
I test fired a few shots out of the stock and all is well.. no leaks and the breech seals perfectly. I filled it to 3000psi and fired its first string over the chrony without any hammer weights fitted.. 105gr cast HP bullet 3000psi: FPS = 921, 929, 916, 896, 881, 840, 829. 1700ish psi end pressure, not bad.. I need to fill my dive bottles to try a full 3500psi fill which should show the top of the curve rising slightly and 10-13 usable shots. I will aim to peak the thread at 900fps so will drop the hammer spring tension slightly too.Realistically this build will be usd a 50-100 yard deer gun so 3 high power even shots is plenty but I will tune the flattest consistent string out of interest. 200ftlbs in a bullpup certainly puts a smile on my face! Just for poops and giggles I put a 44gr .30 cal JSB pellet over the chrono..3000psi fill got 1176fps and it sounded like a .223rem letting rip! These pellets are too small for this bore and the twist is too fast but it was entertaining. Finally got it fully assembled and in the stock with the forward scope mounts set up, Ive just got to do a small job on the trigger linkage tomorrow and its ready for tuning and hope to get some groups on paper.I have backed the power off to a maximum of 915fps to help flatten the string a little more too so I'll also run a few more strings over the chrony.. 
Weight was something I was thinking of throughout this build as it is a deer rifle which means it will be carried through the hills for hours on end. I just weighed it and it is 3.5kg with the scope (7.7lbs).. much lighter than all me other bullpups. Its showing some very good groups at 100 yards too.. shot several groups around 1.5" today with the 105gr HP but they should get even better once my .308 bullet sizing die turns up and I can size this lead a little smaller for this tight match barrel.. I finished it all off yesterday by attaching the new trigger linkage and polishing all the parts ready for anodizing. 
Here's the results of it first 100m field test.. I tested two bullets, my 105gr HP and a 85gr HP 
I shot prone of my rest and there was a strong gusty wind up the gully I was shooting through so I expected a little horizontal patterning, I'm a little over waiting for the weather to clear so I can test this build so pushed on.. 
After getting the scope zeroed at 100 yards I shot the following 5 shot group with the lighter 85gr HPs 
I then topped it up to 3000psi again and shot these 10 shots consecutively of that fill... the wind had really picked up at this point : 
So although a windy and rainy day I have the confidence I can get 5 shots inside an inch at 100 meters on a still fine day with both these bullets... I will try again when the weather settles. : This was the first 5 shots I put on paper at 100m the other day when sighting and tuning.. 
With a huge THANK YOU to my big bore buddies Neil and Danny, I now have a good selection of .30 bullets to test in the pup today.. the sun is shining and there is not a breath of wind.. I will tether it and unleash the thunder on paper.. Heres the line up, Danny made me a double mould cavity with a 85gr HP and a 105gr HP to try too.The first two bullets beside the .30cal JSB pellet are both his designs one has a bigger hollow point than the other. The 5th from the left is his new 105gr HP with a larger hollow point than my one which is an attempt to get a better performing deer bullet. All the rest apart from my duller looking 100gr HP in the middle are Neils bullets. L/H to R/H: 47gr JSB, 85gr HP, 85gr bucket mouth HP, 118gr Flat Point, 105gr HP, 100gr HP, 128gr HP 132gr Spitzer, 158gr Spitzer, 163gr HP, and for a scale reference 8.44gr .177 JSB 18.1gr .22 JSB 
To say it was nice to get out of the house and put 100+ shots through the .30 would be an understatement.. 
Here's the results from several groups from each bullet at 100 yards prone of the rest.. some could be refined with velocity adjustments and different hollow point and sizing options but Im happy with the results I got. 
Top group of the three 
Middle group of the three 
Bottom group of the three 
Two groups from JSBs which are actually far too small for this barrel, they are designed for a .300" bore not my .3074" bore.. they did quite well considering.. 
The clear winners were the 100gr HPs with a 3/4" group! 
Top group 
Bottom group, these new 85gr HPs from Danny consistently printed 1" to 1.5" groups and will be my "go to bullet for lighter game like rabbits and goats as they have better expansion and velocity advantages over the 100gr that I will use on larger game. 
It seems to like the spitzers too, particularly the heavier 158gr which brings the theoretical down range energy up a touch but in real world tests on game the tend to pass through with little terminal damage unless tipped over and sent tumbling by contact with bone.. Still they have a draw card for me and the lower muzzle report, the increase in recoil and loud thump from down range from this heavier pill just seems to feel right in this pup.. the lighter bullets seem softer to shoot but really crack in comparison. I think its time to strip it down for anodizing black then get it out bush ;D This 7.62mm will be a bit of work horse so I didnt spend a lot of time on finishing etc but have plans of some really ornate and unique work on a future build.. I've just got done giving it a registered identity (legal requirement in arms manufacturing here).. but still haven't filled out the "model" box yet.. I'm stuck for a fitting name... 
My mission is to build a lightweight short .30cal big bore PCP from scratch suitable for taking deer and pigs with cast projectiles. 
I have 3 different barrels to test with a range of twist rates and bore diameters for the range of .308 cast projectiles which are in themselves tunable in weight and balance with an adjustable depth hollow point.Ive got my money on the 1-14" twist .307.5 match barrel for the 105gr cast projectiles but also have a 1-18" twist for 80gr -120gr projectiles and a 1-10.5" twist shallow cut microgroove for up to 160gr projectiles. This is the bullet I have chosen, it is a 115gr Lyman bullet 
The mould has been modified into a hollow point that now casts a 95-110gr HP pure lead bullet depending on hollow point depths. 
My goal is to get about 170-180ftlbs from this rifle so although making a bullet for a barrel before the rifle is even built may seem a little backwards but it was a big part of the R&D process for this project and I needed to be very sure there was a bullet that would suit my needs and will be stable at a lower than intended velocity range in one or more of my barrels. I pushed a sized one through the microgroove bore to get a feel of the bore friction and check out the rifling groove cuts in the bullet.. the weight and balance can be fine tuned by altering the hollow point depth and diameter at a later date if needed. 
For the action I cut a piece of 1-1/4" 6061 T6-51 alloy to length and skimmed one side down on the mill to 1" to form the start of the receiver. 
Then I marked the barrel/bolt centre, fitted up my 4 jaw chuck and drilled the pilot hole with a long series 8mm drill bit.. 
Followed by the 1/2" barrel clearance size drill and a 10mm bore for the bolt.I then cut the loading port for the feed tray and the 45 degree chamfers to pretty it up.. 
A little more milling and the basic shaping is done.. It will get a raius to mate up with the cylinder, transfer port and then the bolt and cocking pin cut outs on the underside.. 
A few more components to the list.. the rear plug is yet to be bored out and threaded for an external power adjuster/spring guide and the bolt still needs the probe, O ring groove and handle to be made.The 1-14" twist match barrel has been pre fitted and is ready for the transfer port to be drilled. I have decided to use a modified German made Gecardo trigger because Im too lazy to make one from scratch.. 
Progress has been a little slow this week as I have been a little pre-occupied but I spent a little time on the lathe tonight and made the bolt handle.Because of the heavier hammer springs needed on big bore airguns, there is a trend to have a separate cocking handle directly into the hammer and protruding from the cylinder but I'm not a big fan of that "double action" system. 
This is normally done to lessen the cocking pressure but I have decided to have the hammer cocking of the bolt so it had to be built tough and of a big enough diameter to take extra load After shaping and trimming to length I heated it red hot and put a nice bend in it. It is drilled into the bolts shaft and held tight in place by a recessed 4mm cap screw. 
Next up the high flow valve.. I cut a length of 6061 T6 bar a little longer than I need for the valve but Ill use the other half on the front plug/fill valve.After turning to the inside diameter of the air cylinder I drilled a 5mm pilot hole right through and then parted it off a little over the length it will end up and bored the various port, valve shank and seat internal diameters. 
Once all the internal bores were cut I measured up and drilled the transfer port and then milled a counter sink for a transfer port tube. 
The valve was then slipped into the air cylinder and located it with the transfer tube. Once locked in place I drilled the holes three valve retaining cap screws and countersunk them with a 3/8"mill cutter so the heads of the cap screws are in shear load not the threaded shanks. This "pinning" system is far stronger and safer that threaded end plugs that can strip out and thin the cylinder walls in the threaded sections. 
Back in the lathe for a polish and to cut the O ring grooves 
Then a hole and 1/8" BSP tapered thread to fit a downward facing manometer. 
With a final polish and clean it is ready for assembly. The gauge will get a brass extension to drop it down flush with the walnut stock so I haven't wound it tight into the tapered threads 
A little more progress in the last few days...I decided to use a fill probe rather than a foster fitting to keep the cylinder shorter and tidy so started by turning a fill probe out of a piece of 12mm brass hex rod and milling a blind 8mm hole into another chunk of alloy bar. I wanted to leave it "blind" so the fill port was only visible from the underside of the cylinder and lessen the risk of introducing debris in to the air cylinder and valve etc. 
Once the plug was turned to a nice slip fit into the cylinder and fitted with an O ring I centre drilled the 3/32" air inlet port with a long series drill bit into the 8mm fill probe port and cut a 3mm thread for a countersink head cap screw to create the one way fill valve. 
I assembled the simple one way valve design consisting of a 3mm countersunk cap screw with the thread filed flat on one side snugged down on a 005 O ring and tested/adjusted its flow rate on the dive bottle.. I adjusted it to a restriction pressure of 500psi on a fully open dive bottle valve (3200psi) which will be enough to let air in slowly but completely seal and not let air back out when the probe is removed. Works a charm.. ;D 
The front plug is held in place by three countersunk 12.9 grade cap screws like the valve body again with the heads in shear load not the threads. 
A small mod was needed after initial testing as it was hydraulicing the air in front of the probe which prevented me from pushing it all the way home.. a 1mm vent port drilled into the top side of the fill probe hole sorted that out and allowed the probe to seat all the way home. 
So with both shiny new parts fitted up to the air cylinder I filled it with air and submerged it in a bath of water to check for any leaks then left it over night to check for any leak down.. Sunrise came and the cylinder hasn't dropped any pressure at all. 
With the air cylinder sealed up and the valve working I moved onto fitting the receiver to the cylinder.The transfer port was used to centralise the action on the cylinder which consists of a steel transfer tube compressed between the two parts in blind counter-bores sealed with external O rings. 
Once the transfer port was mated up I drilled the action for four flush fitting 4mm cap screws to lock it all down in place 
After some careful measuring of the hammer travel and total spring compression range I then cut a slot for a cocking pin and mated it up with a second slot in the receiver for the opposing cocking pin/bolt lock up. I have used a much bigger cap screw in the bolt and recessed the head a little as it is subjected to a lot of rearward pressure on discharge, I will also be adding a second smaller "safety screw" on the bolt in case the main locking screw ever failed and tried to exit the action rearward. Its not as much as a safety hazard in a bullpup as it would exit over top of your shoulder and probably leave you with a ringing ear but in a conventional rifle set up the bolt is right in front of your nose :shock: 
The action works smoothly but it does takes quite a bit of effort to compress the hammer spring with the added side loadings of the cocking pins etc so regretfully I think I may have to cut a second slot in the side for a stand alone direct cocking handle. On a positive note: the proportions look pretty sweet in its walnut bullpup stock. The .308 match barrel is currently cut to 23 inches, this cast bullet barrel will be a deer, pig and goat slayer so noise is no issue and will probably stay unsuppressed.. I may make a nice break for the muzzle though. 
Ive had a few emails from guys asking where I got this bullpup stock from, it is one I made a couple of years back for a .25cal Marauder but it is a bit of a cupboard queen at the moment so I have decided to share the stock and the forward scope riser mounts/trigger group with my new .30cal build. So for those of you that missed that build thread here's a brief look at the making the stock.. (sorry to all those that have seen it before) The stock started as a large 6 foot long 2.5" thick slab of 120+ year old black walnut.. I spent about an hour moving the template around the slab trying to envision the finished stock... Im very happy with the chosen piece but wont really see the true grain until I start the shaping. I should have taken a pic of the vast array of router guides and jigging I had set up to get the inletting right, It looked awkward to put it best.. This is my first stock Ive made by hand that is not to be a painted finish so Ive been sooooo careful not to slip with any power tools..that said there's a lot of work to do on it yet so if you see a sudden shift from the idea of polished walnut to a nice olive drab paint job, you can assume Ive muffed it up and had to bog something.... 
I spent about an hour behind the router tonight snorting sawdust and reminiscing my college wood working class...I made a good start on the inletting. Nice snug fit too. 
The scary inletting job is all done, it was a little more complex than I first thought so I'm very relieved to have that stage finished..Another couple of hours behind the router and its starting to take shape..I had to shift the lateral router line down about 10mm due to the thin wood in the scope mount recesses. I wanted to be well clear so not to weaken it or in case they "broke through". It hasn't really changed the look to the original design though, they still look good.I really wanted to stray away from the "chunky" looking bullpup designs commonly seen and endeavour to produce a thinner, slick, light weightand more ergonomic stock. 
I finished cutting the basic outline now and have started roughing out the contours.. Its starting to show some really nice grain now.. 
I've got it all shaped out now which only leaves the sanding part... lots and lots of sanding!I'm really loving the lines of this stock, the grain follows the curves just as I hoped it would whenI laid out the template. I have tried to keep the Eastern European stile lines which I'm quite fond of.Probably not every bodies cup of tea.. but then again nor are bullpups 
A total of 20 hours work and the stock is finished just applied the final coat of oil. with the recoil pad fitted the stock weighs 980 grams. 
...and here's the forward trigger group and scope mount risers I will also be re-using: 
A little more progress in the last few days.. Regretfully I cut another slot in the side of the cylinder to add a stand alone cocking handle, not really my preference but it is much better in terms of smooth operation. it cocks with very little effort now and the cocking handle has added weight to the hammer which may eliminate the need for an internal tuning weight. I have left it quite long but will cut it down a little once I shoot a few strings. I also spent a little time polishing the machining marks out of the various parts, eventually after all testing is done the external alloy parts will be bead blasted and anodized flat black. 
I cut the bolt probe to length and fitted two 007 O rings (cool size huh) :mrgreen: to it, it seals just inside the breach against the receiver itself ant the other sits just outside the transfer port in the barrel. I am a fan of at least two O rings in this position as there's nothing worse than a breech leak on a bullpup.. loud and uncomfortable is the best case scenario and a permanently flared nostril and an eyelid turned inside out is the beginning of something quite dangerous.. 
I have shortened the bolts stroke to just allow a bullet in the loading tray by a second safety cap screw in the bolts shaft and added a sprung ball bearing with a detent in the bolt shank to positively lock it up in the closed position. It all runs very smooth now. 
The 26".308 match barrel started its life on a .308 win target rifle and after the .308 Win chamber section was removed it showed signs of a section of throat erosion which is common and it had a an inch or so or wear at the muzzle end too probably from excessive cleaning from that end.I have trimmed both ends back to remove the bad sections and have ended up with 21" of good strong shiny rifling to work with.. I have added a muzzle break to keep the aesthetics looking right because it ended up an inch and a half shorter than I'd hoped but"accuracy first" 
Thanks to my gunsmith friend for the loan of a piloted .308 tapered throat ream I have the cast bullets slipping in with perfect resistance and a nice gradual taper from the lands to the groove without any angled shoulder to gather lead.This match barrel is tight, it slugs at .3072" so I will be sizing my bullets to .308" not .309" like I had intended for a .308 bore. Ill be very keen to see these on paper.. 
Nearly finished.. I still have to modify the scope riser mounts for this barrel and set up the trigger linkage..It is VERY light and will be a pleasure to carry, it is 30" over all length. 
I test fired a few shots out of the stock and all is well.. no leaks and the breech seals perfectly. I filled it to 3000psi and fired its first string over the chrony without any hammer weights fitted.. 105gr cast HP bullet 3000psi: FPS = 921, 929, 916, 896, 881, 840, 829. 1700ish psi end pressure, not bad.. I need to fill my dive bottles to try a full 3500psi fill which should show the top of the curve rising slightly and 10-13 usable shots. I will aim to peak the thread at 900fps so will drop the hammer spring tension slightly too.Realistically this build will be usd a 50-100 yard deer gun so 3 high power even shots is plenty but I will tune the flattest consistent string out of interest. 200ftlbs in a bullpup certainly puts a smile on my face! Just for poops and giggles I put a 44gr .30 cal JSB pellet over the chrono..3000psi fill got 1176fps and it sounded like a .223rem letting rip! These pellets are too small for this bore and the twist is too fast but it was entertaining. Finally got it fully assembled and in the stock with the forward scope mounts set up, Ive just got to do a small job on the trigger linkage tomorrow and its ready for tuning and hope to get some groups on paper.I have backed the power off to a maximum of 915fps to help flatten the string a little more too so I'll also run a few more strings over the chrony.. 
Weight was something I was thinking of throughout this build as it is a deer rifle which means it will be carried through the hills for hours on end. I just weighed it and it is 3.5kg with the scope (7.7lbs).. much lighter than all me other bullpups. Its showing some very good groups at 100 yards too.. shot several groups around 1.5" today with the 105gr HP but they should get even better once my .308 bullet sizing die turns up and I can size this lead a little smaller for this tight match barrel.. I finished it all off yesterday by attaching the new trigger linkage and polishing all the parts ready for anodizing. 
Here's the results of it first 100m field test.. I tested two bullets, my 105gr HP and a 85gr HP 
I shot prone of my rest and there was a strong gusty wind up the gully I was shooting through so I expected a little horizontal patterning, I'm a little over waiting for the weather to clear so I can test this build so pushed on.. 
After getting the scope zeroed at 100 yards I shot the following 5 shot group with the lighter 85gr HPs 
I then topped it up to 3000psi again and shot these 10 shots consecutively of that fill... the wind had really picked up at this point : 
So although a windy and rainy day I have the confidence I can get 5 shots inside an inch at 100 meters on a still fine day with both these bullets... I will try again when the weather settles. : This was the first 5 shots I put on paper at 100m the other day when sighting and tuning.. 
With a huge THANK YOU to my big bore buddies Neil and Danny, I now have a good selection of .30 bullets to test in the pup today.. the sun is shining and there is not a breath of wind.. I will tether it and unleash the thunder on paper.. Heres the line up, Danny made me a double mould cavity with a 85gr HP and a 105gr HP to try too.The first two bullets beside the .30cal JSB pellet are both his designs one has a bigger hollow point than the other. The 5th from the left is his new 105gr HP with a larger hollow point than my one which is an attempt to get a better performing deer bullet. All the rest apart from my duller looking 100gr HP in the middle are Neils bullets. L/H to R/H: 47gr JSB, 85gr HP, 85gr bucket mouth HP, 118gr Flat Point, 105gr HP, 100gr HP, 128gr HP 132gr Spitzer, 158gr Spitzer, 163gr HP, and for a scale reference 8.44gr .177 JSB 18.1gr .22 JSB 
To say it was nice to get out of the house and put 100+ shots through the .30 would be an understatement.. 
Here's the results from several groups from each bullet at 100 yards prone of the rest.. some could be refined with velocity adjustments and different hollow point and sizing options but Im happy with the results I got. 
Top group of the three 
Middle group of the three 
Bottom group of the three 
Two groups from JSBs which are actually far too small for this barrel, they are designed for a .300" bore not my .3074" bore.. they did quite well considering.. 
The clear winners were the 100gr HPs with a 3/4" group! 
Top group 
Bottom group, these new 85gr HPs from Danny consistently printed 1" to 1.5" groups and will be my "go to bullet for lighter game like rabbits and goats as they have better expansion and velocity advantages over the 100gr that I will use on larger game. 
It seems to like the spitzers too, particularly the heavier 158gr which brings the theoretical down range energy up a touch but in real world tests on game the tend to pass through with little terminal damage unless tipped over and sent tumbling by contact with bone.. Still they have a draw card for me and the lower muzzle report, the increase in recoil and loud thump from down range from this heavier pill just seems to feel right in this pup.. the lighter bullets seem softer to shoot but really crack in comparison. I think its time to strip it down for anodizing black then get it out bush ;D This 7.62mm will be a bit of work horse so I didnt spend a lot of time on finishing etc but have plans of some really ornate and unique work on a future build.. I've just got done giving it a registered identity (legal requirement in arms manufacturing here).. but still haven't filled out the "model" box yet.. I'm stuck for a fitting name... 
lots of time and talent shared. 
