Semi auto PCP pistol

[JPSAXNC]

I have the fill valve mostly made, hopefully I can complete the lower end by the end of the week.

[JPSAXNC]

I had to remake the fill valve, I made the screw pattern wrong. Still making progress.

[sb327]

When are we going to see some progress pics. Sounds like you’re getting close!

Dave

[JPSAXNC]

Getting close to finishing the tube and valves. Today I hope to have the hammer spring  and guide made and see if the valves hold air and the hammer cycles. Maybe some pics this week.

[JPSAXNC]

The tube holds air and fires. The hammer won't cycle yet because there is no back pressure with the open transfer port. The hammer spring was a nightmare, I tried winding a big diameter spring so the tube id. would be the guide but no luck. I used a small diameter spring, only one coil from not being able to cock it. The hammer needs 1.2" of travel, but there is only enough space for the spring guide to be .548 long. I had to place the spring guide in the back of the spring so that just as the spring is starting to kink the guide keys it into it. The trigger pull is a creepy 3-5 lbs. hard to tell exactly. The trigger is very simple, a transfer bar connects the trigger to a vertical lever that moves back to drop the sear. I'll have to lay out a breech for it, I have a barrel.

[JPSAXNC]

I have block for the breech started, and see an issue. I used three screws to hold the valves equally spaced. The two upper screws interfere with the breech block, I'll machine a slot to clear them. I've managed to build myself into a number of boxes, so far I've been able to find a way out.

[subscriber]

James,

That sounds like an exciting way to make things.  Since I learned to use CAD, I prefer to use that to ensure nothing clashes in a complex design.  Of course, I do lots of designs without ever making parts, so you are way ahead.

I prefer using a manual mill and lathe to CNC, so perhaps there is hope for me yet.

Merry Christmas

[JPSAXNC]

Yep, the fun never ends. The breech block is about 4.4" long I was going to use a 6" long 1/16" diameter drill to make a pilot hole through the breech. It broke about 2" in. Today I get to see if I can remove it some how. I already have another bad idea to see if I can get to it. LOL

[subscriber]

Ouch!

[WobblyHand]

Yep, the fun never ends. The breech block is about 4.4" long I was going to use a 6" long 1/16" diameter drill to make a pilot hole through the breech. It broke about 2" in. Today I get to see if I can remove it some how. I already have another bad idea to see if I can get to it. LOL

Ouch.  I've broken a drill in a similar way.  The breech is made out of aluminum?  Maybe you can dissolve the steel bit using warm alum solution? 

Depending on where the drill broke, your choices are pretty limited.  I've never had luck with extractors.  Could try carbide, but if that breaks, good luck.  Another more expensive option is EDM.

I have a different question.  Why such a small hole over such a long distance?  Small drills wander.  Second, you are in gun drill territory, a simple twist drill is really hard to drill a long straight hole.  Your L/D ratio (length/diameter of drill) is 4.4/(1/16) which is 70.4!  Twist drills have a tough time with L/D greater than 10.  Yes, it can be done, but 70?  You need a gun drill, or it's equivalent.  Or you possess some mad machining skills, in which case, I'm in awe.

https://unisig.com/information-and-resources/what-is-deep-hole-drilling/what-is-gun-drilling/
https://www.cnccookbook.com/gun-drilling-bta-drilling/

[JPSAXNC]

The tip of the broken drill was about 2" deep and about 1" of drill was stuck in the aluminum block. I first tried drilling it out with a carbide drill, but the carbide couldn't t cut it. So I picked up the part from the opposite end and drilled down to the broken drill hoping to push it back out, but it wouldn't budge. So I tried using  a length of straight 1/16" music wire as a drift tapping the the broken drill from both ends, finally it looked like it moved about .001, I could feel a slight edge with a pick. So I set it back up and went back in with the carbide drill and was able to push it out . So I finished the bolt probe hole on the mill, and then put it in the four jaw chuck on the lathe to bore the hole for the barrel. The critical part will be to get the depth of the magazine slot right so it's in line with the barrel and the bolt probe.

[WobblyHand]

I think you should buy a lottery ticket with that save!

[JPSAXNC]

I got the magazine slot cut, the depth is right and it's a nice snug fit. Now I'll have to drill screw holes in the tube to mount it. I'm going to use a spare barrel I already had made for my Jet pistol. There's still more work to do.

[JPSAXNC]

The breech is finished, not too much left to do. The overall length is 12.5" the weight is 26.6 oz. I still have to make the bolt probe and do some deburring and finish polishing the inside of the tube. Last I'll have to modify the grip frame to clear the bottom screw on the firing valve, the mod won't be visible from the out side. I have a magazine ordered for it, but it won't come till next Friday. I'll take some pics this week.

[Rob M]

JAMES PICS

image0 (1) by murphyrobert9, on Flickr

image1 by murphyrobert9, on Flickr

[Rob M]

cool project james.. I didnt gather all the details , but looks like a solid design . My concern is the trigger, unless its set up for semi auto , you will have full auto all the time.

[subscriber]

Thanks for the pics, Rob,

The bolt and hammer combination makes it clearer how the semi-auto works:  A projectile is fed from the mag into the barrel past the TP, directly before the hammer smacks the valve stem.

Is that correct, James?  If so, variable friction in feeding could affect the valve strike.  I guess you need to make it feed smoothly, or ensure there is a little more hammer strike than required.

Mentally, this arrangement reminds of an open bolt SMG.  If the mechanism that pushes the bolt back, short strokes, just short of latching the sear, it will go full-auto. 

For normal semi-auto operation, the sear will need a disconnector, that allows one shot per trigger pull.  Trigger release will need to reset the disconnector. Without such a disconnector, you will have full-auto.

[JPSAXNC]

Thanks for posting the pics Rob. The grip frame is  from a Benjamin 422 pistol it's a semi auto so that is covered, contingent on the hammer going back far enough to latch up. I tried to adjust the hammer strike on the valve stem and the bolt probe seating to be simultaneous. So that the mass of the hammer and probe would hold the breech closed for an instant. It's not going to be a power house if it works, I'm hoping for 12fpe. The reservoir is about 51cc's. subscriber, open bolt smg was the basics for the idea.

[subscriber]

James,

So the thrust of air on the probe is what will cycle the action, just like a blowback PB?  In your instance, the inertia of the bolt equals roughly twice that of the combined mass of the probe and hammer, because the parts are moving forward as the firing cycle starts.  This assumes that the valve opens before the probe runs out of stoke.  Else, the 2X value become 1 x, which is probably plenty for a low powered airgun shooting a light pellet (compared to a PB).

As long as the probe is still sealing the breech it would not matter too much if it moves before the pellet has left the muzzle.  Yes, the expansion volume would increase slightly as the pellet travels down the barrel.  I am sure that Bob could calculate the effect on air efficiency, but I doubt it matters very much.

What you don't want is the hammer re-cocking violently, because that may be  hard on the parts, and cause sharp recoil.  The latter may be fun, as the pellet will be gone from the barrel, so it will not be hold sensitive like a spring piston airgun.  It may affect the amount of time taken to settle for the next shot, compared to a Leshiy 2, for example; shooting at equivalent power.

[JPSAXNC]

Hi subscriber, There's a 1/16" diameter hole drilled through the valve along side the stem hole that will feed air into the o-ring sealed hammer to cycle the action. Surprisingly The push back on the probe is very small. When I was fiddling with the Velox pistol I removed the spring that pushes the bolt forward to see how much push back there was on the probe, and at 11 fpe. in .22 the probe didn't jump back. But the probe is attached to the hammer and another part. So there was a lot of mass for the probe to move, but still at 11fpe. and the probe being the same diameter as the pellet. I expected the probe to jump back but it moved only slightly. I wonder about the hammer recoil to, I'll  find out in a couple of days.