PCP cartridge, Mark 2

[LeMaudit]

Here is a revision for my PCP cartridge. I followed your advices, removed the O-ring, increase the pressure by planning to use a shock-pump, decrease the chamber volume to less than 1 inch3 so I don’t have to pump too much (I’m lazy), and also adjusted the sizes to real material available at my closest Home-Depot and bicycle parts store The tubing is now a standard 1 inch diameter with 7/8 internal hole (supposed to be 1200 psi rating), and 7/8” rubber washers with 3/8” hole seems readily available. A photograph of the Schrader valve I found at the bicycle parts store is on the drawing and seems perfect for what I want to do.

There was also a potential flaw in the previous design. Some links in the piston where loose, which was intended to avoid a vent hole. But I think it could potentially be dangerous if things are not aligned correctly when the pressure is build up, an accidental discharge could occurs. Of course I don’t want that! So I put back vent holes and a single piece piston. GGDT don’t show much performance lost anyway. And the mechanism will be more able to stand the pressure now increased to 400psi, which is about 200 pound (95 kg) for my internal 7/8” diameter. The piston is a bit more complicated to machine, but the rest is simpler so that’s okay for me. I also removed the internal O-ring for sliding the piston as I think I can machine a very good fit easily, and as soon as the firing starts there is no more pressure lost anyway thanks to the back rubber washer that seals everything.

So, here is a drawing again. If some of you care to comment and criticize, I’ll be more than happy. Unless you find a flaw, I think I will build this one for real.

With 400psi, which drop to 375 with the increase of the internal volume by 1/8” length when fired, GGDT gives me for a 8 inch length barrel a 14 ft*lb of energy, and 343 ft/s

I do intend to make two 12” extensions for the barrel, to change the handgun into a medium and long rifle. GGDT again gives me those figures:

20 inch barrel (8 + 12 extension) = 23 ft*lb, 430 ft/s
32 inch barrel (8 + 12 extension x2) = 27 ft*lb, 464 ft/s
Which is good as I’m still under the 500 ft/s Canadian law, but with great power! Well, at least I think it’s great

Anyway, back to the drawing board to prepare the rest of the design. I still don’t have the workshop installed, but when it’s done I promise you’ll see some shiny brass pieces

[jackssmirkingrevenge]

Looking good I like the magnet idea.

Just one thing, if you're not using an o-ring in the piston then the vent holes are unnecessary, air will still leak past the piston even if it's a very close fit.

[LeMaudit]

Really? Okay, it's easy to test without holes, then add them later than the other way around

I figured the magnet will be easy to hold the ball bearing, but also to remove it (with a bigger magnet) so I can disarm the cartridges if necessary. With a rubber sleeve as planned first, the firing would have being mandatory.

Thanks again for all the advices.

[jackssmirkingrevenge]

Really? Okay, it's easy to test without holes, then add them later than the other way around

Trust me, you won't need them. Have a look at my prototype, you'll notice that the piston is a simple epoxy cylinder. Tight fit of course as it's cast in the actual tube it travels in, but it stil lets enough air to leak past to fill the chamber.

[LeMaudit]

I have no doubt the air would leak, but my first feeling would have being that it would not leak fast enough to create an equilibrium between the two sides of the piston in between two pumping. But I'm sure you're right of course I'm just discovering all this stuff.

Great fun this prototype of yours!!!