Semi auto PCP pistol

[Rob M]

safety factor at 3600 should be 4.7 to 1 for the tube itself .. as for the 3 screws , were assuming 90.000psi  for hardened screws so the factor is in a similar range .. I messed up somewhere for the screws , I'll try again. ( I'm getting 6.98  to 1 for the screws ... this assumes only the thread is in sheer plane .elastic deformity of the holes will happen first most likely.. something to check after numerous cycles.

[JPSAXNC]

I'm charging the gun to 3000psi., no issues so far. I have made some progress trying to flatten the shot string. Making changes to the valve didn't help because the hammer was hitting the valve preventing the valve from self regulating. Since the bolt probe is fixed to the hammer, I wanted the hammer travel to be such that the pellet would be far enough forward that it didn't block the transfer port.  I had to make the valve stem longer to get more unrestricted valve action. I have no idea where the pellet is when the hammer strikes the valve now, but the velocity is up because the valve is opening further.

[subscriber]

James, your tenacity is impressive.

[JPSAXNC]

Hi subscriber, It's just that at the end of the day the valve is a simple knock open valve, regardless of how many systems are tied together. I built myself into a box with the distance the hammer and probe had to travel. But I have way out, I'll have sorted in a couple of days.

[Rob M]

CORRECTION !!WARNING !!! the safety factor on the screws at 3600 psi is 2-1 .. I knew I screwed up somewhere , and now I figured it out.. at 3000 psi its still 2.3 which isn't too bad.. upgrading to 10-32 makes a huge difference

[JPSAXNC]

Thanks Rob, I'm filing to 3000psi. now. I will keep an eye on it for creeping. Like you mentioned before, the aluminum screw holes will probably show some elongation. So far there's no change.

[JPSAXNC]

Getting closer. I wanted 21 shots enough for 3 magazines @ 12fpe. start pressure 3000psi. end pressure 1800psi. .22 cphp's  638 631 632 633 636 628 641 651 637 634 628 633 630 638 626 616 627 621 626 624 625. There's a lot of pressure still left in the gun, it seems to me that I should be able to get more shots. It could be that the hammer is still hitting the valve when the pressure drops far enough. I will mark the face of the hammer with red crayon and see if it leaves a mark on the valve?

[JPSAXNC]

Here's a shot string 24 shots  608 626 632 635 626 641 636 641 630 625 635 639 640 634 630 635 619 623 632 629 629 626 626 624. I'm taking a break from tuning the string, I have a set of sights from a CP1M pistol I'm mounting on this pistol. It's fun to shoot and very quite with the magazine in place, a lot of sound comes from the open breech with no mag. in place.

[JPSAXNC]

I've put about 800-900 shots through this pistol, most have been singled loaded. Shooting though the seven shot magazines, the velocity of the shots increases as the shot count in the magazines decreases. The amount of spring tension on star wheel has a very noticeable effect on velocity because of the drag it puts on the bolt probe.  Here's an example fastest shot through the magazine, 646 fastest shot single load, 668.

[subscriber]

A lot of factors!

[JPSAXNC]

Yes, I wonder if some complaints blamed on supposed faulty regulators is due to star wheel drag?

[JPSAXNC]

I have an idea that might over come the star wheel drag. I'll try a heavier hammer with more momentum, it might not make a difference though because when the gun is cocked the tip of the bolt probe is almost touching the back of the pellet skirt. So the hammer doesn't get a chance to build up any speed, but the added weight might help.?

[JPSAXNC]

A heavier hammer didn't make any difference. I removed the star wheel from the magazine so I could check the magazine aliment in the pistol. The shots with and without the magazine were very close. I noticed the star wheel spring was wound from heavier wire than it needs to be, I'll see if i can wind a lighter spring.

[JPSAXNC]

The stock magazine spring was wound from .023 wire, I wound a new spring from .012 wire. It made a noticeable difference, The spring tension at the 7th hole is the same as the 1st hole, barely any tension at all. But there is still a difference in velocity. The bolt probe has a little free play in the breech, so even the lightest pressure from the star wheel can push the tip of the probe downwards, I think causing it to clip the front side of the magazine. I'll make a tighter fitting bolt probe and see if there's a difference.

[JPSAXNC]

Still working on magazine issues. I had to lower the magazine slot on the breech to try and get better alignment. For automatic cycling the alignment has to be as perfect as possible, any amount of drag slows the system and affects velocity. I'll see if it's better today.

[JPSAXNC]

Made some progress with the magazines. But a weird thing happened the first two shots were normal, the third and fourth shots were louder and there was no reading on the chronograph. I took the magazine out and the pellet was still in the firing position in the magazine. Since the hammer and probe are pinned together it's impossible for the hammer to move far enough forward to strike the valve stem, without also cambering a pellet. The only thing I can think of is that when the gun fired the pellet went backwards, back into the magazine. I don't believe that, but it's the only thing I can think of, it happened 4 times. I'll take the pistol apart latter today.

[sb327]

It sounds as though the magazine didn’t advance until it was pulled from the gun. I’m not sure how much visibility you have of how it’s indexed while it’s in the gun so this may obviously not be the case…?

Dave

[JPSAXNC]

Thanks, that makes sense with the much weaker mag spring that I put in the mag. My brain tends to make mountains out of mole hills.

[JPSAXNC]

Basically the open bolt design has been a success. I have to make a new sear for it though. The factory sear was a crudely stamped part that was lobsided so the hammer only contacted one outside edge. I had made a new sear for it, but it got damaged because I was fiddling with the sear spring tension and set it to light by mistake so the hammer was chipping away at it because it wasn't coming back up fast enough. I'm really pleased with the overall performance it's better than I expected.

[subscriber]

James, it has been a fun education to watch you refine this pistol.