preloaded pneumatic cartridge

[frankrede]

I think it would work better if it was shorter with a long piston to eliminate all that dead volume in the slim inner tube. Wasn't that your original plan?
3/4 inch copper and various fittings makes a perfect marble shell.

I believe the round has to long so it can have more volume in its chamber

[Hawkeye]

Yes, but it would have basically the same volume if a 'toolies' piston spanned the length of the cartridge. The difference being that the air would hit the projectile immediately .
Going back through the posts I realize that this was the original idea. Why was it scrapped, apart from messing up the first one?

[jackssmirkingrevenge]

Going back through the posts I realize that this was the original idea. Why was it scrapped, apart from messing up the first one?

That's basically the reason - epoxy can be a pretty depressing material to work with, first of all due to curing times you can never build something quickly, and if when you're done it doesn't work, you can basically just bin it and start over - In this case, I preferred to stick with something I knew would work rather than go through the heartache of perfecting the original design.

I have to say though, it's pretty *profanities* powerful for something with a barrel length of less than 1", I'll post some videos later on

[jackssmirkingrevenge]

First (barrelless!) video uploaded, check out this thread

[joannaardway]

Well, I spent half an hour poking around at GGDT. Hitting the balance between valve bore and chamber volume for maximum power is difficult...

An early design idea comes to me as something like a pump action (or lever action for that matter) shotgun with an underbarrel magazine.

Good design work could rejuvenate my old plans to create a launcher based on one of the Winchester 1887 shotgun (as trivia, the similar 1901 model featured in Terminator 2)

[jackssmirkingrevenge]

An early design idea comes to me as something like a pump action (or lever action for that matter) shotgun with an underbarrel magazine.

That would complicate matters slightly, in the sense that you'd have to machine a groove at the base of the cartridge body to give something for the loading/extracting mechanism to grab on to.

Having cartridges eject on firing or by racking a pump would look cool in equal measures, I think I've gone for the slightly easier route.

I'm currently casting a breech out of epoxy so tomorrow I can evaluate how well the cartridge will be blow back on firing - I'm a little concerned because unlike my other pneumatic and combustion cartridge prototypes, this design requires the forward impetus of a firing pin striking the schrader to fire, which will offer greater resistance to the cartridge being blown back. I'm hoping that 400-odd psi will be enough to overcome this.

What I like about self-contained cartridges is the modular aspect of the design, in the sense that if something goes wrong with the mechanism, you can just make another cart instead of a whole new launcher, which is particularly useful for the "permanent" nature of epoxy construction.

[joannaardway]

That would complicate matters slightly, in the sense that you'd have to machine a groove at the base of the cartridge body to give something for the loading/extracting mechanism to grab on to.

There is an easier option than machining a rim - magnets. Or if I choose to use 22mm copper to make the cartridge (which is likely), then I could just cut a narrow ring off a coupler and attach that in place.

When the air volume available is so small, I'd be happier to use a manual reload, rather than one that drains off power from what little is available in the first place.

Anyway, thanks for the inspiration. Now I need to sit and think.

[jackssmirkingrevenge]

When the air volume available is so small, I'd be happier to use a manual reload, rather than one that drains off power from what little is available in the first place.

Fair point, the 1/4" ply test shot was made with the cartridge firmly attached to the barrel - it remains to be seen how much the power will be retained if the cartridge is allowed to slide out of the breech, hopefully I'll be able to do some testing tomorrow.

I hope I lubed up everything correctly, otherwise it'll just be one solid mess

[Hawkeye]

Could you set it up so that the hammer/bolt/firing pin strikes the schrader and pushes the whole barrel and cart forward slightly as well so that it bounces slightly. Then the rebound may push back with sufficient force to pop the cart out.
If you had the barrel floating in a track with a large soft spring around it or in front of it perhaps. That would make it easy to slot the cart in if you wanted to manually load them.
If the system had a bit more natural "play" to it ,it may be easier to get the cart to do what you want it to.
The firing pin could depress the schrader valve at the moment the barrel bottoms out on its forward travel. As it rebounds, the cart could hit a small ejector in typical fashion.
I'm not sure how you plan on hitting the stem of the schrader. That may take some precision.

[jackssmirkingrevenge]

Timing would be a big issue, I'm not sure how practical that would be - though instead of the bolt moving forward to hit the schrader, I could have the whole barrel/breech assembly move back, Barrett 50 cal style, hmmm.

As to hitting the schrader stem, it's mounted in the centre of the cartridge so it shouldn't be a problem.

[jackssmirkingrevenge]

Well, here's the cast epoxy breech attached to the barrel - pretty, isn't it?

The "grooves" that appear on the surface are from the grease I used to lube the mould. That section of syringe tube wrapped in masking tape is part of the mould and should have come off but I'm not holding my breath.

The cartridge slides in and out quite smoothly, to be tested later today.

Edit: I made a slight miscalculation, the breech was cast with the cartridge unpressurised and it slides pretty smoothly, but once I take it up to pressure it actually expands slightly and jammed in place, so I had to do some sanding.

Fired a few test shots, a little disappointing. The cart attempts to eject but with nowhere near the force that my combustion cartridge had, it doesn't look like a semi-auto launcher is worth pursuing.

I do however have a working cartridge, and that leaves a number of manual/spring ejection possiblities, and I'm beginning to be inclined towards this one:

[youtube][/youtube]

It won't be very realistic, my cartridge is bigger that the massive 4 bore round, lt alone your average 12 gauge - perhaps a single barrel will suffice... but two barrels would double the coolness factor. Decisions, decisions...

[joannaardway]

I've been thinking.

A little design work has created concepts loosely based on a 12 gauge round (which should be suitable for paintballs or BB "buckshot"), a .50 BMG round (1/2" ball bearings), and the NATO 5.56mm round (in 6mm BB size).

I'm most tempted by the 12 gauge round, even if it will be closer to 4 1/2" in length than the more ideal 2.5" to 3", whereas the others are roughly right in their dimensions - the concept of a shotgun launcher seems best to me. The extra length of the round will limit the capacity of any tubular magazine, but it should be cool anyway.

[jackssmirkingrevenge]

You'll have your work cut out feeding from a tube mag, have a look here - also, at such small chamber volumes, I hope you got yourself a shock pump

[joannaardway]

Yeah, but why do anything if it's easy?

[spud yeti]

Yeah, but why do anything if it's easy?

Because it may give good results, even if its easy
Or to boost your ego