Need opinions/advice on a bb gun design

[Brian the brain]

Thanks a lot Brian! I've been looking at your rifles for a few weeks now, hoping to make something simillar at some point. What I don't understand is how do you refill a coaxial valve for multiple shots? Is there another valve somewhere that I can't see that when opened, refils the coax? Thanks!

The valve does not dump all of the pressure.
The hammervalve is knocked open only for a split second, dumping the pilot volume and then after is shuts the main piston resets automatically.
An extra supply of air in the shoulderstock makes the total volume big enough to get multiple shots.

[gpopovic]

Thanks!

I think I get it, will try to make a drawing to clear it in my head Do you have an concept drawing perhaps?

[Brian the brain]

Very basic principle.
in a coax pistonvalve the piston seals against the barrel naturally because of the difference in pressure.Inside the barrel is normal atmospheric pressure, all around the exposed sides of the piston is high pressure.

If pressure behind the piston is dropped, the pressure around the barrel will push the piston back,opening the path to the barrel.

With a snug -but not airtight- piston fit...the piston will reset under pressure when the pilot valve closes because the pilot area is refilled almost instantly, while air is rushing out the barrel, creating a low pressure zone in front of the piston while pressure rises behind it,
Effectively sucking the piston back onto the barrel.Asif you are using a vacuumcleaner to suck a snookerball out of the pocket.

The pilotvalve needs to open fast and then almost instantly close again to keep as much of the pressure in the launcher, yet release enough out the barrel to lauch the projectile at full force.

With a hammervalve as the pilot..( I use a partially counterbalanced one for higher pressures)and a very small pilot volume this will give you multishot capacity with much more power than a hammervalve could provide on its own.

This works even without the extra airsupply coming in from the back.Important to know it is not required for the multishot effect.
I just needed more volume for more shots and I could hide an extra reservoir in the shoulderstock.

I guess you could do wonders with this concept!

[gpopovic]

Thanks a lot Brian, both for the drawing and the kind words!

It's actually simpler than I thought, great! I'm already thinking about the tubes that I already have at the shop,and the ways to hit the pilot... Some drawings are to follow!

[gpopovic]

Finally finishing the pcp. The barrel is threaded and epoxied in, and the reservoire is finally sealed. I will file it flush today and epoxy coat the surface. The action still needs some minor work, mostly due to my fingers being unable to place the 5.5mm pellet into the barrel from the first try

pcp montaza1 by Goran P, on Flickr

[gpopovic]

Brian, I drew up something based on your concept, and tried to place it in the form of one of my favourite single shots, Remington Rolling block. Billie Dixon model, 30' barrel, completely to scale, just the barrel is thinner than the original, which is 30mm diameter. Mine is 22mm, and should be counterbalanced by the reservoire in the stock, so it's not too front end heavy.

Would it work? The inner barrel would be 10x2mm, which with some minor work works great with the 5.5mm lead pellets.

btbinspiredrolingblock by Goran P, on Flickr

[Brian the brain]

For 5.5mm a pistonvalve is overkill..

You could use a hammervalve for that.
The benefit of a piston valve is you can use much bigger calibers.
I recently put a .50 barrel on Lil'Shat.

[gpopovic]

Overkill is not a problem 5.5 is cheap and readily available, and I would primarily use it for plinking. 10x2mm that I can get is supposed to be ID6mm, but it never is, it's closer to 5 really, so I enlarge it a bit. and its just right for 5,5mm.
These tubes are rated to 9bar I think, ~130+ psi.

BTW, what's your wall thickness on the inner tube? I planned this with both wall thicknesses 2mm.

On the other side, I epoxied the reservoire as planned, and should machine the action on the pcp a little to allow better access for the fingers when placing the pellet, hopefully tomorrow. Will make some pics if I manage.

[Brian the brain]

Barrel walls are just 1.1 mm,copper, the walls of the outer tube however are 3- and then some-mm seamless steel..

[gpopovic]

I guess my 22x2 with car compressor would be quite OK then.
Got my bandsaw back in the working order with some nice new bands, 6mm wide, supposed to cut even mild steel, but I don't suppose I'll try that
Looking around the shop and some pieces of cherry look suspiciously like a RB stock, we'll see...

[gpopovic]

Some more work done on the receiver, drilled the port from the bottom, and I am toying with the idea of it's plug being also a screw that's holding it to the stock. It would be hidden under the guard probably.

pcp montaza2 by Goran P, on Flickr

When I start a new project from the scratch, I'll make a separate thread, I think this is OK for now?

BTW, I got asked to replicate a broken stock for a really old russian breakbarrel, side-by-side. It's still in one piece, but with a few nasty cracks and a poor attempt of a repair. I can fix it, but he'll probably prefere a brand new one, perhaps with some twists of his own. I'll repair the original anyway, and it will hang nicely on my shop wall if he doesn't want it