Still a balance between the increased time the ammo is pushed by the air versus overcoming the frictio of the barrel.

Actually a homemade check valve with minimal inlet would be a better choice than a commercial one. The input from a handpump is pretty minimal so it only needs enough room for the air to "hiss" past the seal. Even if it locked open or failed completely it shouldn't allow enough air back out to form ...

With a 3/4 inch copper pump I can reach 300 psi by basically using the pump like a shock pump. A 1/2 inch copper pump will be perfect for a hand held high pressure launcher. With a little creativity the pump could be incorporated like a pump action in the forestock where the pump handle sleeves arou...

Use a floating o-ring on the piston head. Of course the configuration shown will work as long as you actually have the parts to make it as shown. I see a lot of vague diagrams that show parts sleeved neatly together with no signs of threads etc. There are usually practical limits to getting the fini...

Chances are the male is the problem. Did you drop it and perhaps chip or damage the part that seals against the o-ring?

Yep, Use thread on fittings.

I just meant that the upper part of the stock doesn't really look very face friendly.

I'm trying to picture how this thing is going to be held...

Only needed for a forward travelling hammer? Like in the Airforce Condor? lol Neither of the diagrams look like they have enough hammer travel but the secondary lever at least gives the possibility. With the short travel and single trigger you're going to need a heavier spring which will make it har...

The idea is to have the trigger in an ergonomic position. You could move it forward but then you're reaching up the forestock to trigger it.
The secondary sear lever is pretty common I think.

The secondary lever is necessary to allow the hammer to travel further and get a better swing at the valve pin.

A short drill bit extender makes a good inline hammer. I have found that it's a slide fit in the smaller diameter copper.

It's probably the air is free factor for pneumatics. The world we live in today probably scares people away from messing with anything explosive especially when the legality of it is dubious in many locations. Driving around with an empty propane cylinder can carry something like a half million fine...

I have an old "Guns Annual" or something like that that shows a diagram of the Bate airgun.

I'm not sure how complicated this has to be. You just need a solid cylinder of metal that's a slide fit in your metal tube that will house it. Not that hard to find. A brass nipple can house the striker and you can use an end cap with a hole drilled in it so that the striker can be pulled back by ha...