I'll be making thousands of these cheap, as soon as PTP builds them for me.
I was fiddling with this idea for a while. Not a whole lot you can do with a single shot marker, but the idea was to make it simple. When PTP had their garage sale, I thought I might do this for the pile-of-pooh build. All but one of the parts I ordered ended up scattered across the united states via the giant hole the USPS put in the mailer. I finally got another barrel. I really wanted to build a pair of these for dueling, but I figured I had a lot of prototyping to do in the first place. It is a low pressure (~225 psi) balanced, retreating spool valve. It is a muzzle loader.
The trigger is just a brass rod stick trigger, and the stock is a piece of 2x4. The "receiver" is a PTP automag foregrip with autococker threads with an extra 1/4" bore, and something like a 5/16" bore in the back, er top. The fill fitting is a HPA bottle check valve.
This is in the fired position
This is in the ready position
This shows the breech plug that fits in the barrel (standard cocker threads), the pointy spool, and the preload spring.
The breech plug is coned on the ball side.
These are all of the parts except for the brass trigger/sear. I fill it with an HPA bottle with an ergo reg and gauge hooked to a quick disconnect and slide check. You connect it, slide it up, slide back and disconnect. It's not very loud and shoots reballs around the bedroom very well, when the wife is not home.
The 225 psi is what I ended up with for about 280 fps in early testing. Now that I have my precision frame, I need to get some first strikes for it. There's a bodarc tree that I think will be the donor of my real stock, and I have the brass for a trigger guard.
Here's a better pic of the arrangement.
and the sear, dremel'ed and drilled from a piece of 1/4" brass square,
and the fill station, as it is
so, a bit of finish work left to do.