Another Custom Marauder build; advice needed

[phillips4100]

First shots today!

The only OEM parts here are the breech, air tube and trigger...

• 26" TJ Barrel 1/14 twist from mountain air customs
• carbon fiber shroud from Neil Clauge
• Donny FL Sumo
• Huma regulator (large plenum version)
• Cothran Valve
• Boyd's pro Varmint laminate stock
• Bushnell Forge 3 - 18 X 50 scope
• Huma replacement fill port with gage

My goal with this build is to shoot heavier slugs (50 + grains). I'd like to get them in the 900fps range for long distance shooting (no practical reason. I'm just having fun)

I took my first shots today. Below I'll post some 50 and 100 yard groups; so far the accuracy is promising, but I'll need to do more to get the power and efficiency to where I'd like. The 52grn boat-tails are running right at 800 FPS. Reg is set to 150 bar and I have drilled the transfer ports to .187. Using the stock hammer and spring, with my own free flight device the velocity stays the same no matter how much preload I put on the spring. I've got a hill airgun hammer that's a little lighter weight and I've purchased a TSS from a forum member which might give me a little more adjustment capability. The problem is in order to get the power I'm shooting for I think I'll need to open up the transfer port even further. I'm Using a Teflon tube (bought from Tim hill) that set up fits perfectly into the existing transfer port design and gives the largest diameter I can get given the amount of room available. Does anyone have tricks or advice on how to open up the marauder ports even further?

50 YDS

100 YDS

 

[Motorhead]

Fun stuff I can relate to VERY custom m-rods.

Bigger transfer path takes you machining the air tube transfer tube hole larger, as well the breech.

In my HP build done last month ( A .25 cal ) it used a spin off JSAR Gen 2 valve of my own manufacture. Found with it BIGGER in transfer DID NOT gain more power and actually had equal power and more shots with a strangled in outlet/transfer area. valve opened with very light hammer and SSG controlling spring ( Zero preload on poppet stem )



MORE POWER requires MORE PRESSURE delivered to the projectile ... simple physics.



Good looking rig right there !!

 

[terrible_t]

You can purchase reamers in .001" increments, drill .005-.015" undersize of the reamer you want the diameter of your hole to be. Run the reamer at a slow RPM and use some sort of light oil for lubrication. Feed into your hole and out slowly for good surface finish. It would be best in a drill press, you can use a hand drill if you really have to but most likely it won't go in very straight and you'll end up opening the hole up larger than what the reamer diameter actually is. You can also buy gauge pins to check the diameter. Calipers are okay but for measuring a hole diameter they are only accurate within a +/-.002" since they have flats ground onto the measuring edge.

NICE looking project! Very cool.

https://www.mcmaster.com/reamers/high-speed-steel-round-shank-reamers-10/

 

[Blejda]

Looks better then high end airguns with that stock and it can shoot so I think job well done. 

 

[2L8]

I knew Motorhead would be one of the first to comment . Listen to his advice and you will have the Marauder shooter of your dreams.

 

[Vetmx]

When swinging for the fence chasing power, efficiency usually goes right out the window. That’s when a complete power plant redesign is in order. In your case you may be pushing past the limits of your power plants design. You can’t even expect it’s best accuracy if it isn’t operating efficiently. You might want to get your guns efficiency where you like it in its current state. Then you will truly know where you are at and if your goals are obtainable. What diameter are your slugs sized to? What are you lubing them with? Is your barrel supported out near the front? If not, you could be shooting those slugs through a whip.

 

[phillips4100]

Thanks for the feedback. next step is to get a lighter hammer and spring so I can dial it back a little and get more efficient. I'm also going to up the regulaor to 160 before considering any major surgury to get the transfor ports wider....

Vetmx: you asked if the barel was suppored up front. Conventional wisdom has always been to free-float the barrel. do you think that's not the case here?

 

[Motorhead]

Just for some info that may be able to use ?

My Recent High output .25 "Motorauder" .. Lol See: https://www.airgunnation.com/topic/when-is-a-marauder-no-longer-one-it-becomes-this-exotic-monster/

 

[WHITEFANG]

MH knows his guns and he is one that shares much for free. So I would take notes when he shares. MH is A great addition to any forum. As the link he attached that gun is tricked out. The Marauder is a great platform to work.

Thanks MH!

 

[Motorhead]

As another bit of info spoken of earlier .. that of larger transfer ports with balanced valves NOT necessarily being the better choice READ THIS and especially the last technical post by Scotchmo : https://www.airgunnation.com/topic/advanced-tuning-for-the-jsar-raptors-as-well-ss-2-balanced-valves/

This info may very well also be relative with the Cothrin Machine valve your using.



Scott S

 

[phillips4100]

Yes, I ran across your post while working on my gun and inquired about your hammer design. Very cool build. I wish I had the skill (and time) do do more of the custom work myself. I only tinker so it's great to have folks like you share their ideas and expertise.

 

[kkarmical]

Beautiful build, but curious why 52 BT instead of 55.5 HP or 43.5HP?

The 52gr BT are the only one of NSA slugs I do not use.

Just had too many online exchanges with PB long range shooters, that say boattail don't really matter until past 800 yrds, and then marginally.

That's why I went with the 43.5 & 55.5gr's for my heavy metal, and personally love the 55.5gr's at 920fps, the sweet spot I've found with them out of my talonp.

I tuned my Mrod back down to 42fpe, where it performs best.

If I was using your gun I would use the 43.5gr HP or the 38.5gr HP, where they would move a little faster and probably out perform those 52grs..even the 36gr domes which btw I love this slug in the 60 fpe range, amazing performance.

On a side note, how are you lubricating your slugs?

 

[Vetmx]

I don’t trust a free floated barrel that isn’t threaded into the receiver. Unless it’s super short. A slip fit with a couple grub screws and orings is not a solid zero tolerance setup. Your barrel looks extra long then you have a can hanging on the end of it. The more power you add, the more you are going to shake that thing. Ten guys can chime in about how accurate their long floating barrels are but then the conversation can move to POI shifts and we can see how that goes. On my slug guns, I lock that barrel down. Still would like to know what you are sizing your slugs to and what lube.

 

[Motorhead]

Long and tensioned via a threaded barrel muzzle with a screw on LDC. This compresses the shroud tube while stretching the barrel within. 3 set screw sets into pockets makes it Super stiff !!

 

[phillips4100]

I don’t trust a free floated barrel that isn’t threaded into the receiver. Unless it’s super short. A slip fit with a couple grub screws and orings is not a solid zero tolerance setup. Your barrel looks extra long then you have a can hanging on the end of it. The more power you add, the more you are going to shake that thing. Ten guys can chime in about how accurate their long floating barrels are but then the conversation can move to POI shifts and we can see how that goes. On my slug guns, I lock that barrel down. Still would like to know what you are sizing your slugs to and what lube.

interesting, I'll have to do some experimenting. as for sizing and lube I havent had enough time with the gun yet. I have .249 and .248 siing dies that I'll run some slugs through and try out this weekend. Usually I'll spay some power lube in a ziploc bag to lighly coat them before running through the die but thats about it.

 

[Vetmx]

If your barrel is a TJ, .248 and .249 are going to be too small. That’s a choked barrel slug size. Lube your slugs with silicone oil before you shoot them. As Motorhead mentioned, tensioned barrels are great but in your case it’s just easier to use a band or whatever Mrods use. No orings. Shim it, then tap it for a grub screw. Make it ridged. 

 

[mackeral5]

to get 50gr in the 900's you will likely need to abandon the regulator, or perhaps double the plenum size, and increase porting to .200 or so...also, the Cothran valve doesn't respond to hammer spring adjustment like a traditional valve. Hammer spring basically turns the Cothran on or off. this can be debated, but directionally it is true. Dwell varies very little in relation to hammer strike, thus changes in hammer strike do not necessarily affect velocity. . pressure is more directly related to velocity. 



That is one sweet looking rig you have here. I look forward to seeing more about how it works for you.

 

[Guest]

That is one sweet looking marauder! I would love to have something like that with shiny freefloating bullbarrel! 

 

[phillips4100]

It's been a while but making some progress.... shot this group today at 100 yards with the Nielsen 52 grain boat tails. I've been testing a lot of pellets and most fall into 1.5" to 2.5" groups at 100 but the boat tails are the most consistent.

Here is the string from that group, nice and tight but I'm only getting 7 or 8 shots on the regulator from 3100 down to 2200 PSI. That's an efficiency in the range of .85 so I think I could be doing better. Anyone have experience with the Cothran Valve and how to get it a little more efficient?

Description: NSA BT 52 .249
Bullet Weight (gr): 52.000
# FPS FT-LBS
5 886 90.65
4 881 89.63
3 881 89.63
2 880 89.43
1 881 89.63
Average: 881.80
StdDev: 2.39
Min: 880
Max: 886
Spread: 6

 

[addertooth]

If that is a generation II Marauder with a stock bolt... it partially covers/impedes flow from the transfer port into the barrel. (from the picture your bolt looks changed, but I did not see it in the parts list). I re-contoured my bolt (where it rests over the transfer port when the bolt is forward and locked) to get better total flow with a lower pressure. My goals were different than yours. I was going for a long shot string with much lighter pellets at 40 fpe. (42 shots at 40 fpe with a 1.8 percent maximum variation between the fastest and slowest shot). I radiused the **inside** of the bore at the transfer port, with the only major removal facing towards the end of the bore. You have to stop the radius before the back of the pellet (round) as you don't want that pressure applied to the side of your pellet (round). Think of everything which makes the air change direction of travel, and try to make that transition as smooth as possible. Sharp edges (abrupt changes in direction) are the enemy of air flow. I used the Huma regulator which had the larger plenum volume as well. The valve was the HILL which had matched 0.187 hole and the lightest return spring they had. The hammer was the super light anodized aluminum model from Hill, with a TSS spring assembly.