QB78D Carbine Project

[UCChris]

Stumbled across this thread about a week back and it reminded me of my love for short carbines. Anyone who has known me since I first got into airguns around 2013 knows that I get the urge to chop down barrels almost the moment get a gun. So I started to think about a project to make a little .177 carbine.

I happened to receive a QB78D in .177 from a fellow AGN member, so I decided this would be the base for the project. It's not the lightest or the most ergonomic, but my love of airguns was shaped by Crosman franken-gun projects back in the day, so this should be a fun, nostalgic project.

The QB78D got a Safepac, which I'm quite impressed by. Of course it has less volume than fitting a modified factory cap, but the simplicity is quite delightful. It was very simple to install and doesn't appear to have any leaks. The rifle came with an Archer XP needle probe bolt and Prod TP seal. Other than that, the gun is stock internally. Still cock in close, no trigger work done.

I started by filling the Safepac to the stated 2k psi maximum. Shot a string over the chronograph and it was clear that the valve was partially locked at that pressure. Not really a surprise since the valve was designed to work with CO2 pressures. Dropped the fill pressure to 1800 and got the following string.

Seems to still be starting with a bit of valve lock, no? Also, the bell curve is more like a mountain peak than anything. It's been a hot minute since I've tuned a gun that wasn't regulated. My understanding is that I need to A) drop the starting fill pressure a bit more and B) either restrict the transfer port or get a lighter hammer strike to flatten the string a bit. Also, I was unable to determine the ending PSI, as I couldn't hear the "click" when filling with my hand pump or tank, so I figure it must be ending at a pretty low PSI.

I'm going to chop this barrel, but before I do that, I would like to get the factory config shooting a decent 5 shots with a reasonable ES. Realistically, this gun will end up as a sub-30 yard woods walker. My goal is 11-12 fpe for a minimum of 5 shots. We'll see how that goes, as I plan to lop off 5.5"-6" of barrel.

Here's a crappy photo of the gun, on top of my dirty freezer, as it sits now. Thank you to @Flintsack and @Boostcreep for inspiring this project!

 

[shawnlyndy]

Very interesting.can’t wait to hear and see your final results.

 

[Dr. Kralenstein]

Nice rifle you got there

I would go the other direction, add more pressure and a stiffer, shorter hammer spring and run an adjuster. Those safepacs I've had up to 3500 psi on the 22xx platforms. Im not recommending you do that high of pressure, but 30 fpe for a few shots out of a 14" 2240 was cooking!

Also, take that felt thingy out of the valve and clean it up for better flow. Could also go with a stiffer valve spring to keep cocking force down and would help you get a smoother curve.

I almost kept the rifle and put a safepac on it, but have too many other gun projects going so I figured sell it before it went on the chopping block!

 

[UCChris]

Funny enough, @Boostcreep , as you were replying, I was doing some testing with increasing the hammer spring tension. I found a couple of 1/16" washers that fit the spring guide pretty well. Because the spring guide has a curve where the rod meets the base, the washers stand a bit proud. Like so:

Two of the washers + the gap ended up with .14" of increased preload. I filled the gun to 2000psi and got the following Korean cliff:

Pulled one of the washers, which left a total preload of .08". Filled to 2k and got the following string:

And if we look at the string as a graph:

Unfortunately, I got lazy and didn't deburr the barrel grub screw or transfer port hole when I first took the barrel out, so my orings have gotten sliced up. I'm not sure if that slower second shot was from the oring damage or if the gun is just inconsistent at the moment. I thought I had orings for the QB, but it appears I either don't or misplaced them. So I'll order some of those.

In the meantime, it appears the peak of the bell curve is just a couple shots under 2k psi. I know the creator of the Safepac has hydro tested the QB model to 4400psi. At a 2200psi fill, that puts us at a minimum of a 2:1 safety margin. If I recall from reading Bob's threads back in the day, a 3:1 safety margin is ideal for PCP's. I'm willing to try a 2200psi fill to see what the curve looks like, but 3500psi seems a bit sketchy.

Also, I forgot how rough these QB's feel in stock form. I've been spoiled by having owned some professionally tuned and modified QB's. Looking forward to getting this one smoothed out.

 

[Dr. Kralenstein]

@UUChris 3500PSI IS SKETCHY but I did it for science! I still have all my digits so that's my experience lol.

I don't know if you can restrict the exhaust port on the valve or make a smaller breech seal. That would level out the string as well. More than a couple ways to skin that cat!

Come to think of it, the 3:1 safety margin that Bob was talking about was for the valve retaining screws for like a disco valve. When you use the stock air tubes and pin the valves, your putting the pressure on the heads of the screws and they push against the air tube, effectively loading them in shear. The safepac is all sealed to the stock valve, and the walls are super thick on the safepac. I would think they will hold alot of pressure. Mike at TKO hydrotested a 2240 roll stamp tube and it didn't fail until like 10K psi, and it failed at the front threads, not the stamping or the screw heads. Be safe and cautious, but I think you can squeak it up to 2500-3000 no problem. I would lol.

 

[UCChris]

@UUChris 3500PSI IS SKETCHY but I did it for science! I still have all my digits so that's my experience lol.

I don't know if you can restrict the exhaust port on the valve or make a smaller breech seal. That would level out the string as well. More than a couple ways to skin that cat!

Come to think of it, the 3:1 safety margin that Bob was talking about was for the valve retaining screws for like a disco valve. When you use the stock air tubes and pin the valves, your putting the pressure on the heads of the screws and they push against the air tube, effectively loading them in shear. The safepac is all sealed to the stock valve, and the walls are super thick on the safepac. I would think they will hold alot of pressure. Mike at TKO hydrotested a 2240 roll stamp tube and it didn't fail until like 10K psi, and it failed at the front threads, not the stamping or the screw heads. Be safe and cautious, but I think you can squeak it up to 2500-3000 no problem. I would lol.

I filled to 2200psi and things weren't much better. The first five shots were with partial valve lock and in the low 600's. Then, starting with shot one in the following string, the velocity shot right up to the 900's and came down real quick.

So it seems my options are either heavier hammer spring and see if I can get a flatter curve with the peak in the 2k psi area. However, I think I'm going to clip the hammer spring just a bit and see what a lower pressure tune looks like.

I'm not confident enough to try and fill the transfer port and drill it out smaller, so that's not an option at the moment.

 

[Dr. Kralenstein]

I filled to 2200psi and things weren't much better. The first five shots were with partial valve lock and in the low 600's. Then, starting with shot one in the following string, the velocity shot right up to the 900's and came down real quick.

View attachment 313270

So it seems my options are either heavier hammer spring and see if I can get a flatter curve with the peak in the 2k psi area. However, I think I'm going to clip the hammer spring just a bit and see what a lower pressure tune looks like.

I'm not confident enough to try and fill the transfer port and drill it out smaller, so that's not an option at the moment.

Another method is get a set screw and tap for it. Thread the exhaust port on the valve, and then drill a hole in the socket head hole of the set screw. Thats how alot of the restricted fps guns are set up in Canada. They just block off the transfer port, you can usually just unscrew them.

What I had planned for the qb78 safepac was a 3000 psi fill, smaller transfer port, and stiffer spring with a velocity adjuster. Would get a good amount of shots at a decent power level I'd think. I was getting 14 full power 14 fpe shots from my 1740 set up that way with an 18" barrel. I'd think 12 fpe would be easy to do on the qb78.

 

[UCChris]

Another method is get a set screw and tap for it. Thread the exhaust port on the valve, and then drill a hole in the socket head hole of the set screw. Thats how alot of the restricted fps guns are set up in Canada. They just block off the transfer port, you can usually just unscrew them.

What I had planned for the qb78 safepac was a 3000 psi fill, smaller transfer port, and stiffer spring with a velocity adjuster. Would get a good amount of shots at a decent power level I'd think. I was getting 14 full power 14 fpe shots from my 1740 set up that way with an 18" barrel. I'd think 12 fpe would be easy to do on the qb78.

Oh, that's a really good idea! I'll measure the stock exhaust port and see which size tap to go with.

Edit: Looks like an M5 tap is perfect for the transfer port. I'll have the tap on Thursday and will resume testing this weekend.

 

[nervoustrig]

Hi Chris, I didn’t see your thread until just now...subscribing to follow.

 

[Dr. Kralenstein]

Oh, that's a really good idea! I'll measure the stock exhaust port and see which size tap to go with.

Edit: Looks like an M5 tap is perfect for the transfer port. I'll have the tap on Thursday and will resume testing this weekend.

M5 is pretty small, but you should be able to get a set screw in there with an 1/8" or smaller hole. Just make sure you don't round out the socket hole so you can still remove the screw.
Can always drill the exhaust port a little bigger and go up in size to give more port options with the screw method.

 

[UCChris]

M5 is pretty small, but you should be able to get a set screw in there with an 1/8" or smaller hole. Just make sure you don't round out the socket hole so you can still remove the screw.
Can always drill the exhaust port a little bigger and go up in size to give more port options with the screw method.

I chose M5 so I wouldn't have to drill the current transfer port at all, simply need to thread it. This reduces the chance of me buggering it up and I can always drill larger and tap for something bigger.

Hi Chris, I didn’t see your thread until just now...subscribing to follow.

I was wondering when the QB king himself would show up. Seeing as I hope this gun is someday in your shop for spa treatment, please forgive me for the monkey with a wrench treatment I'm about to put this gun through.

 

[Hw9720]

Looking for QB info. I have one of the new black thumb hole Beeman QB78's. I also have a nice stach of 88 gram Co2 tanks. I was looking around trying to find the screw on adapther for the tanks (With no luck). While I was looking I found vague reference to the new QB tubes that they were higher pressure than the old ones. Any one know what the info was trying to referance?
tim

 

[UCChris]

Looking for QB info. I have one of the new black thumb hole Beeman QB78's. I also have a nice stach of 88 gram Co2 tanks. I was looking around trying to find the screw on adapther for the tanks (With no luck). While I was looking I found vague reference to the new QB tubes that they were higher pressure than the old ones. Any one know what the info was trying to referance?
tim

Not really sure what they would be referring to. CO2 self regulates its pressure at a given temperature (probably worded that wrong, but you get the idea).

I would think your best bet for 88 gram tanks would actually be a QB79 with a paintball to 88 gram adapter.

 

[Hw9720]

More like the tube was stronger and would work with hpa just by adding a quick fill end cap.
Didn't some one make a direct 88 go to Co2 tube adapter?

Tim

 

[Dr. Kralenstein]

More like the tube was stronger and would work with hpa just by adding a quick fill end cap.
Didn't some one make a direct 88 go to Co2 tube adapter?

Tim

I heard that too, but I would definitely anchor the valve with 2 extra high strength screws. Should be able to find a bulk fill valve for one or possibly use a discovery fill adapter just pinned in place like jow a drop block gets done on a QB.

 

[nervoustrig]

Regarding the grub screw transfer port reduction, I assume we're actually talking about reducing the valve's exhaust port. There isn't enough material in the air tube or receiver to tap a thread.

Also, most grub screws will be a high grade steel...drilling through one might be an adventure. A typical HSS bit will probably just burn away the tip. An M35 cobalt bit might do the trick. A carbide bit will cut it like butter but only in a supremely rigid setup. The slightest chatter or lateral stress and the bit will shatter like glass.

What I would suggest is is take a low grade machine screw and make your own grub screw. Many of the Philips head screws are soft enough to drill with your common HSS bits. Center drill it to the desired size of your port. Then use the thinnest emery wheel in a Dremel to cut a slot across it so you can turn it with a flat-head screwdriver. Then cut it to length.

If that sounds like too much trouble, get back to me after you exhaust your library of curse words while drilling out a hex drive grub screw.

They make brass grub screws and perhaps aluminum as well so that's another possible approach.

 

[Dr. Kralenstein]

Regarding the grub screw transfer port reduction, I assume we're actually talking about reducing the valve's exhaust port. There isn't enough material in the air tube or receiver to tap a thread.

Also, most grub screws will be a high grade steel...drilling through one might be an adventure. A typical HSS bit will probably just burn away the tip. An M35 cobalt bit might do the trick. A carbide bit will cut it like butter but only in a supremely rigid setup. The slightest chatter or lateral stress and the bit will shatter like glass.

What I would suggest is is take a low grade machine screw and make your own grub screw. Many of the Philips head screws are soft enough to drill with your common HSS bits. Center drill it to the desired size of your port. Then use the thinnest emery wheel in a Dremel to cut a slot across it so you can turn it with a flat-head screwdriver. Then cut it to length.

If that sounds like too much trouble, get back to me after you exhaust your library of curse words while drilling out a hex drive grub screw.

They make brass grub screws and perhaps aluminum as well so that's another possible approach.

True I didnt think about that. I've had good luck drilling the black oxide type of set screws, but I like your machine screw idea.

Another option would be to get some small delrin or brass rod the same OD as the T port, and cut to length. Drill it to desired sizes, and would have material enough to make a few different ones and for all the messed up ones you'll be making!

Does anyone know if the transfer port sleeves from the crosman guns will work? I know the seals work because that's what comes with the Archer tune kit. If the metal sleeve bushing thing would fit, possibly have to recess the valve and/or barrel, you could get a bunch of challenger T ports and drill those out. I've used those for restricted ports in alot of 13xx/22xx guns.

 

[nervoustrig]

Also, I was unable to determine the ending PSI, as I couldn't hear the "click" when filling with my hand pump or tank

FWIW that probably means the check valve has a return spring. Unless there is something really hinky about the design of the fill port, the spring is totally unnecessary. But not just unnecessary, its presence means you cannot drain the gun by barely cracking the bleed valve on your hand pump. So you may want to take a look the next time you have it apart.

 

[nervoustrig]

Just as a point of reference, I've made a few minimalist QB78 PCP conversions and it's definitely possible to get a dozen or so shots at 12fpe in .177 cal on the small OEM tube without exceeding a 1500psi fill pressure. Easier in .22 cal. Though bear in mind I'm talking about leaving the barrel at full length, as contrasted with Chris's project where the barrel will be shortened.

By minimalist I mean swapping the end cap to a Foster fill and pressurizing the OEM tube (i.e. no Hipac).

So here's one example...this is one of the synthetic-stocked models (QB78S) I set up to be a lightweight squirrel gun. It gives 13 shots at a smidge over 12fpe which is still about a half dozen more shots that I would need on the most exciting woods walk of my life. Efficiency in this case is above average on account of making a new poppet and lightweight hammer but something like 10 shots at 12fpe would be a reasonable expectation with the OEM parts and some tweaking of hammer spring tension.

 

[UCChris]

The safepac is all sealed to the stock valve, and the walls are super thick on the safepac. I would think they will hold alot of pressure.

I've never dealt with a system that didn't put load on the valve retention screws, so this didn't click with me at first. I've just come to realize that if you drilled a hole in the stock, and had a couple extra Safepacs and QB valves, you could actually hotswap the Safepacs with a single Allen key. You'd have to figure out a way to keep the TP seal in place and carrying a small guppy bottle makes more financial sense, but it's a fun idea.

@nervoustrig You mean in the male portion of the Safepac? There's a block near the valve threads in the Safepac for the valve spring to interface with, so I'd have to access anything in the Foster fitting by removing it. Other than bleeding the air down and knowing my ending PSI, any other upsides to removing it?

Also, thank you for always freely sharing your knowledge; the airgun community is far better for it. Now I know that I can shoot for a lower fill pressure. Obviously it won't be the same without your poppet/hammer mods, but it'll be something to play with for now.