QB78D Carbine Project

[UCChris]

Didn't have time to dig through my spring box, but I did find another few spacers that fit. With the factory hammer spring in cock-on-close configuration, .26" of additional preload results in coil bind just before the bolt closes. I have it set to .21" of additional preload and the bolt closes fine. The trigger pull is now pretty dang heavy, but actually crisper due to the increased pressure on the sear. I will fill to 3k tomorrow and shoot with the .05" TP.

 

[Dr. Kralenstein]

Didn't have time to dig through my spring box, but I did find another few spacers that fit. With the factory hammer spring in cock-on-close configuration, .26" of additional preload results in coil bind just before the bolt closes. I have it set to .21" of additional preload and the bolt closes fine. The trigger pull is now pretty dang heavy, but actually crisper due to the increased pressure on the sear. I will fill to 3k tomorrow and shoot with the .05" TP.

Take some measurements of the spring next time you got it apart and see if you can match the OD with something with thicker wire diameter at your local hardware store. Get one longer and cut down til you get where you need. Need a nice hard quick blip on the poppet stem. I bet the stock spring is giving some bounce and you're losing air.

 

[UCChris]

Take some measurements of the spring next time you got it apart and see if you can match the OD with something with thicker wire diameter at your local hardware store. Get one longer and cut down til you get where you need. Need a nice hard quick blip on the poppet stem. I bet the stock spring is giving some bounce and you're losing air.

Good thinking. There's no discernable bounce in the sound signature, but that doesn't mean there isn't any.

 

[Dr. Kralenstein]

Good thinking. There's no discernable bounce in the sound signature, but that doesn't mean there isn't any.

It didn't seem like it to me either with co2, but definitely wouldn't hurt to eliminate that possibility. Don't know if the qb is known for bounce like the crosman 22xx are. They sound like a door stopper when they get low or don't have a charge.

 

[nervoustrig]

According to my notes, the QB hammer spring is 0.33" OD, 2.35" L, 0.047" wire, 20 coils.

Chris, if you want to convert to cock-on-open, remove the rear cocking pin and insert a spacer (fixed or adjustable) behind the spring guide.

Adjustable (e.g. RVA) is nice of course but for a quick and dirty test, you can use a fixed one and iterate until you get the result you want. You don't have to fully disassemble anymore the way you've had to when adding washers behind the spring. All you have to do is remove the rear trigger frame screw and pop out the plug at the end of the tube. Now you're at the spring guide where you can add or remove spacers.

One approach I've seen others do is stack washers in there until they achieve the result they want, then take them out and measure and cut a spent CO2 cartridge to the same length and use that as the spacer.

 

[UCChris]

According to my notes, the QB hammer spring is 0.33" OD, 2.35" L, 0.047" wire, 20 coils.

Chris, if you want to convert to cock-on-open, remove the rear cocking pin and insert a spacer (fixed or adjustable) behind the spring guide.

Adjustable (e.g. RVA) is nice of course but for a quick and dirty test, you can use a fixed one and iterate until you get the result you want. You don't have to fully disassemble anymore the way you've had to when adding washers behind the spring. All you have to do is remove the rear trigger frame screw and pop out the plug at the end of the tube. Now you're at the spring guide where you can add or remove spacers.

One approach I've seen others do is stack washers in there until they achieve the result they want, then take them out and measure and cut a spent CO2 cartridge to the same length and use that as the spacer.

Thank you Jason! I was rereading the thread from the 30 fpe .22 you made me in 2015. I saw that you mentioned the same steps for thecock on close mod, but I was under the impression that the hammer needed to be drilled or something. I will convert it!

It didn't seem like it to me either with co2, but definitely wouldn't hurt to eliminate that possibility. Don't know if the qb is known for bounce like the crosman 22xx are. They sound like a door stopper when they get low or don't have a charge.

Well, .21" preload and 3k fill was pretty underwhelming with the .05" port. The first 15-ish shots were under 300 fps and not registered by my chrono. Then we got this string.

Shot count: 25
Low: 378
Hi: 713
Avg: 631
STD Dev: 71.4
Spread: 335.0

12/18/22 1:37:02 PM,378
12/18/22 1:37:14 PM,568
12/18/22 1:37:28 PM,608
12/18/22 1:37:39 PM,537
12/18/22 1:37:53 PM,640
12/18/22 1:38:08 PM,641
12/18/22 1:38:20 PM,670
12/18/22 1:38:31 PM,676
12/18/22 1:38:43 PM,685
12/18/22 1:38:59 PM,709
12/18/22 1:39:08 PM,713
12/18/22 1:39:20 PM,705
12/18/22 1:39:35 PM,703
12/18/22 1:39:49 PM,689
12/18/22 1:40:08 PM,690
12/18/22 1:40:16 PM,668
12/18/22 1:40:27 PM,652
12/18/22 1:40:36 PM,649
12/18/22 1:40:47 PM,637
12/18/22 1:40:56 PM,629
12/18/22 1:41:07 PM,615
12/18/22 1:41:21 PM,592
12/18/22 1:41:29 PM,587
12/18/22 1:41:36 PM,576
12/18/22 1:41:44 PM,565

All that resulted in like 7-8 shots in an acceptable spread just under 12 fpe.

 

[Dr. Kralenstein]

Thank you Jason! I was rereading the thread from the 30 fpe .22 you made me in 2015. I saw that you mentioned the cock on close mod, I was under the impression that the hammer needed to be drilled or something. I will convert it!



Well, .21" preload and 3k fill was pretty underwhelming with the .05" port. The first 15-ish shots were under 300 fps and not registered by my chrono. Then we got this string.

Shot count: 25
Low: 378
Hi: 713
Avg: 631
STD Dev: 71.4
Spread: 335.0

12/18/22 1:37:02 PM,378
12/18/22 1:37:14 PM,568
12/18/22 1:37:28 PM,608
12/18/22 1:37:39 PM,537
12/18/22 1:37:53 PM,640
12/18/22 1:38:08 PM,641
12/18/22 1:38:20 PM,670
12/18/22 1:38:31 PM,676
12/18/22 1:38:43 PM,685
12/18/22 1:38:59 PM,709
12/18/22 1:39:08 PM,713
12/18/22 1:39:20 PM,705
12/18/22 1:39:35 PM,703
12/18/22 1:39:49 PM,689
12/18/22 1:40:08 PM,690
12/18/22 1:40:16 PM,668
12/18/22 1:40:27 PM,652
12/18/22 1:40:36 PM,649
12/18/22 1:40:47 PM,637
12/18/22 1:40:56 PM,629
12/18/22 1:41:07 PM,615
12/18/22 1:41:21 PM,592
12/18/22 1:41:29 PM,587
12/18/22 1:41:36 PM,576
12/18/22 1:41:44 PM,565

View attachment 314815

View attachment 314816

All that resulted in like 7-8 shots in an acceptable spread just under 12 fpe.

Starting to get it leveled out though

 

[nervoustrig]

I was under the impression that the hammer needed to be drilled or something.

Oh yeah you're right. Relocating the cocking pin in the hammer may be necessary to leave room to place a pellet in the loading tray. You can probably hold the bolt back by hand just for experimentation purposes but ultimately you'd want to either relocate the pin or cut a rear downslot to latch the bolt out of the way.

 

[Dr. Kralenstein]

Oh yeah you're right. Relocating the cocking pin in the hammer may be necessary to leave room to place a pellet in the loading tray. You can probably hold the bolt back by hand just for experimentation purposes but ultimately you'd want to either relocate the pin or cut a rear downslot to latch the bolt out of the way.

That would be just like the gauntlet breech. Has a notch on both ends of the bolt slot.

 

[nervoustrig]

That would be just like the gauntlet breech. Has a notch on both ends of the bolt slot.

Yes exactly. I used one on my QB HPA conversions for a while and it works fine...becomes part of muscle memory so quickly that it's not a nuisance but still I've come to prefer relocating the pin.

For reference, the topmost picture represents the mod done to the OEM hammer.

 

[UCChris]

My 3D printer is down at the moment, else I would just print up a variety of spacers to play with. I was able to find a spacer that fit, but it was .81" and the hammer would not catch the sear, I presume due to coil bind. I also found a spacer that was .31" and the hammer would catch, but there was no spring pressure to release the hammer. So it appears it's a bit specific with the .6" spacing quoted by Jason. I'll either work on my 3D printer today or grab some tubing to cut down.

 

[UCChris]

Well, I think I may have just been being stupid, cause I've got a .81" spacer behind the spring guide and .25" of spacers on the guide rod behind the spring. Filled to 1500 psi and got a gentle slope down starting at 607. Had to run back to work, but I'll try a higher fill after work.

 

[UCChris]

Alright, been monkeying around with this in my spare time. I've got a ~15" barrel on it and the cock on open mod done. Filled to 2300 psi, first four shots were not read (so under 275 fps) and then this happened:

Shot count: 20
Low: 373
Hi: 579
Avg: 542
STD Dev: 53.0
Spread: 206.0

12/21/22 12:07:01 PM,560
12/21/22 12:07:10 PM,407
12/21/22 12:07:47 PM,373
12/21/22 12:07:56 PM,516
12/21/22 12:08:04 PM,539
12/21/22 12:08:15 PM,547
12/21/22 12:08:26 PM,557
12/21/22 12:08:34 PM,562
12/21/22 12:08:41 PM,568
12/21/22 12:08:49 PM,576
12/21/22 12:09:01 PM,579
12/21/22 12:09:09 PM,573
12/21/22 12:09:17 PM,575
12/21/22 12:09:28 PM,571
12/21/22 12:09:38 PM,569
12/21/22 12:09:48 PM,565
12/21/22 12:09:56 PM,560
12/21/22 12:10:06 PM,552
12/21/22 12:10:18 PM,546
12/21/22 12:10:25 PM,535

I don't understand two things:

1) There's always a middle of the road shot (shot #1 - 560fps) followed by a few slow shots (shots #2&3). And then the bell curve starts going.

2) The number of shots before the peak velocity is always fewer than the shots after the peak.

I trimmed the shot string to this:

Good for 14 shots in a ~5% ES and HW30 power levels. Good enough for shooting garage mice/chipmunks. There are 5 shots before the peak velocity and 8 shots after in that string, which illustrates my point. All I'm saying is regulated guns > unregulated guns. I think I'm done with tuning until this potentially goes to Jason for trigger work and a lightweight hammer.

Also, I got to thinking and I have a 13ci tank with a regulator that creeps a bit. I wonder if I could cobble together parts such that I could use it as a tether tank if I ever wanted to plink with this gun. I wouldn't imagine the regulator creep would matter much as long as the regulator is set somewhere in the middle of the unregulated string.

I've still got some fun to have with an LDC I'm making and the sighting system I want to set up. More on that later!

 

[Dr. Kralenstein]

Alright, been monkeying around with this in my spare time. I've got a ~15" barrel on it and the cock on open mod done. Filled to 2300 psi, first four shots were not read (so under 275 fps) and then this happened:

Shot count: 20
Low: 373
Hi: 579
Avg: 542
STD Dev: 53.0
Spread: 206.0

12/21/22 12:07:01 PM,560
12/21/22 12:07:10 PM,407
12/21/22 12:07:47 PM,373
12/21/22 12:07:56 PM,516
12/21/22 12:08:04 PM,539
12/21/22 12:08:15 PM,547
12/21/22 12:08:26 PM,557
12/21/22 12:08:34 PM,562
12/21/22 12:08:41 PM,568
12/21/22 12:08:49 PM,576
12/21/22 12:09:01 PM,579
12/21/22 12:09:09 PM,573
12/21/22 12:09:17 PM,575
12/21/22 12:09:28 PM,571
12/21/22 12:09:38 PM,569
12/21/22 12:09:48 PM,565
12/21/22 12:09:56 PM,560
12/21/22 12:10:06 PM,552
12/21/22 12:10:18 PM,546
12/21/22 12:10:25 PM,535

I don't understand two things:

1) There's always a middle of the road shot (shot #1 - 560fps) followed by a few slow shots (shots #2&3). And then the bell curve starts going.

2) The number of shots before the peak velocity is always fewer than the shots after the peak.

I trimmed the shot string to this:

View attachment 315679

View attachment 315680

Good for 14 shots in a ~5% ES and HW30 power levels. Good enough for shooting garage mice/chipmunks. There are 5 shots before the peak velocity and 8 shots after in that string, which illustrates my point. All I'm saying is regulated guns > unregulated guns. I think I'm done with tuning until this potentially goes to Jason for trigger work and a lightweight hammer.

Also, I got to thinking and I have a 13ci tank with a regulator that creeps a bit. I wonder if I could cobble together parts such that I could use it as a tether tank if I ever wanted to plink with this gun. I wouldn't imagine the regulator creep would matter much as long as the regulator is set somewhere in the middle of the unregulated string.

I've still got some fun to have with an LDC I'm making and the sighting system I want to set up. More on that later!

Could always gut the regulator bonnet and run full bottle pressure. I sometimes run my 3L tank when testing a gun. Just fill it to my gun's peak pressure and shoot til it starts to drop.

 

[UCChris]

Could always gut the regulator bonnet and run full bottle pressure. I sometimes run my 3L tank when testing a gun. Just fill it to my gun's peak pressure and shoot til it starts to drop.

Intriguing idea. I was thinking about how to run a bottle at full pressure in case the plenum space for my Prod build isn't large enough. Thanks!

 

[UCChris]

Could always gut the regulator bonnet and run full bottle pressure. I sometimes run my 3L tank when testing a gun. Just fill it to my gun's peak pressure and shoot til it starts to drop.

Got thinking about this for a different gun. My wife has a Prod bullpup with a Huma reg in the tube. I want to get a tank adapter that screws in where the fill assembly is and run a bottle at 3k for additional capacity. You have to replace the 1.8k reg burst disk right?

 

[Dr. Kralenstein]

Got thinking about this for a different gun. My wife has a Prod bullpup with a Huma reg in the tube. I want to get a tank adapter that screws in where the fill assembly is and run a bottle at 3k for additional capacity. You have to replace the 1.8k reg burst disk right?

I would think so. I usually just remove the burst disks and replace the bleeder plug with an 1/8" npt or m10x1, whatever they decided to thread the hole with that day lol.

 

[nervoustrig]

I usually just remove the burst disks and replace the bleeder plug...

What is the bleeder plug?

 

[Dr. Kralenstein]

What is the bleeder plug?

The plug that's over the burst disk, they have a little bleed hole in them for when the disk pops. I use them to bleed the air on my tube guns.

I didn't know what to call it lol.

 

[nervoustrig]

Ah okay, thanks. I just refer to the whole thing as the burst disk...the vast majority of the ones I've seen are a 1-piece construction but I'm familiar with the type with a replaceable diaphragm piece so I think that's what you're describing.

The other thing that had me confused is that you referenced a thread type of 1/8" NPT or M10x1 to plug them. Paintball regulators use what are called unified burst disks and they have a 3/8"-24 thread. So if you're plugging them with a 1/8" NPT or M10x1, which are indeed a similar size and therefore may start to engage the threads in the regulator body, the result is a potentially unsafe condition.

Please, please be careful.

Another thing to consider is whether the bonnet is rated for 3000psi. As far as I know, most of them are but one example that is not is the Ninja Pro v2...the type with an indexable bonnet so you can clock the fill port or gauge to your preferred location. I forget what Ninja quoted as the maximum safe working pressure but suffice it to say 3000psi is too much.