SpudFiles

What do you all think of a well-sealed piston that sits in your pneumatic gun's barrel. It would theoretically be airtight, well lubricated and have some mechanism* to stop it launching out the front of your gun like the projectile.

The projectile would sit in front of the piston. This would negate any need for a sabot and increase efficiency by disallowing air to "slip past" your projectile. I was mostly thinking of this for marble-gun applications, as they don't seal great.

*The mechanism to stop the piston from shooting off into the distance would obviously be the main issue in this design. Assuming it was almost airtight, the piston would be blocking off any excess pressure in your chamber by the time it reached the muzzle. To combat this, one might want to drill some holes towards the front of the barrel to bleed off a little pressure. This only reduces the stress on the piston, but does not stop it from exiting the barrel. An obstruction at the muzzle would only jam the bullet, too - so I'm still thinking about some way to contain the piston...

Any thoughts? Is this a reasonable idea assuming we can think of some way to contain the piston?

If my ramblings don't help you in visualizing the idea, let me know and I'll MS-Paint you up a simple diagram.

Nah I don't think its worth while you'd have to vent the piston. and their is a lot of displaced energy in friction and this would only work if the barrel had near equal volume to the chamber even then its going to be hard to stop the piston

maybe you could use a piston to propel a large caliber projectile with a smaller chamber and valve system

You've just described a nailgun

The system is used on impact testing cannon to prevent the sabot from impacting the target and affecting the results of the test.

The Russians also use a similar system in some cartridges like the SP-4 made for the PSS pistol in order to prevent propellant gasses from escaping and thus providing a very quiet shot.

You need a very good way of arresting the piston however, involving springs and lots of rubber - personally though I think a piston that's going to survive the impact will have to be very heavy and have a negative influence on performance, as well as causing a lot of fatigue on the system. The forces of momentum involved are very significant, I don't know if PVC alone will be up to the task.

to get the same effect make a bunch of cheap hotglue pistons and attach them to the projectiles. The original idea could work but it probably isnt worth it. It could be fun though because you could use the same concept to convert your gun into a massive pneumatic hammer(think of the IMPACT hammer from unreal tournament)

oh yeah I remember thoses russian cartridges jack does a good job at explaining things and gives a lot of diagrams

Bah, the piston would be needing to be replaced frequently becase of the slaming into the stop even if there was a bumper. Good thinking though.

iknowmy3tables wrote:jack does a good job at explaining things and gives a lot of diagrams

Yeah, we got bored of testing the burst-disk pneumatic we improvised from cam-lock fittings at work and I have unlimited access to the internet from my desk

I tried this stuff not long ago under the exact same name: http://www.uksgc.co.uk/forums/index.php?showtopic=932

Waste of time really, all I got was vastly reduced energy in the projectiles.

Hotwired wrote:Waste of time really, all I got was vastly reduced energy in the projectiles.

That's better than me, after a few shots of my prototype the barrel cracked and sailed off into the distance at an alarming rate of knots

Hotwired wrote:I tried this stuff not long ago under the exact same name: http://www.uksgc.co.uk/forums/index.php?showtopic=932

Actually, it's not the same idea, there are no floating pistons here - essentially the idea being proposed is simply a sabot that doesn't actually leave the barrel.

On a slight tangent, I'm not surprised your theories didn't work. Essentially the same principle used in their light gas gun, it might be feasible for NASA to try out, with solid propellant giving the piston its boost and hydrogen as a medium, but scaling it down to compressed air changes the parameters somewhat.

To avoid having to have a huge barrel and awkward sabot, I had a reusable piston device on my horseshoe launcher. As you can see, there is a large amount of barrel dedicated to stopping the piston (everything after the holes)

There is a pneumatic baseball pitching machine that uses what is essentially a reusable sabot that stays in the barrel. If you really want to try it you may try to find out what they used. Look up the patent and/or go to a store and try to see the sabot.

LOL all I can think of is "Close only counts in horseshoes and hand grenades"

Since the gas energy is going to have to accelerate the piston as well as the projectile regardless, you might as well have the piston leave the barrel and put a parachute or some arrester on the back of it so it doesn't go too far. Although you're right it would maximize air contact with the projectile, but they have this new invention called wadding . It's a nifty novel concept, but no matter how you try to stop the piston, you're going to destroy your cannon, it's just a matter of how long.

jackssmirkingrevenge wrote:

Hotwired wrote:I tried this stuff not long ago under the exact same name: http://www.uksgc.co.uk/forums/index.php?showtopic=932

Actually, it's not the same idea, there are no floating pistons here - essentially the idea being proposed is simply a sabot that doesn't actually leave the barrel.

One of the ones I used did trap the piston:


The trapped air slowly seeped out around the piston but on one occasion the barrel was blown off the cannon and impaled itself in the ground.

Oh well. It was worth a thought! I suppose the only decent solution is to find projectiles that exactly match your barrel. (I'm not keen on sabots).

Edit: Hotwired, how was the glue for your gun? Any issues so far? Are you concerned about safety in the long term?

What you created there is essentially similar to a spring piston air rifle - it would have been interesting to compare performance if the projectile was attached directly to the piston, eliminating the smaller barrel, or indeed to just have the smaller barrel using the same pressure and a conventional valve.

Deactivated 1 wrote: Edit: Hotwired, how was the glue for your gun? Any issues so far? Are you concerned about safety in the long term?

Do you mean the JustForCopper stuff?

Its working fine, no leaks or cracks, I don't feel particularly concerned about it. The cannon is small enough to get minimal flexing around any of the glued joins so the chance of the glue cracking is pretty low. If by some chance it does it won't be a catastrophic failure, just a loud hiss (what chance is there of glue failing all round a joint at the same time?).

I can also reglue it with a blowtorch (turns the glue to powder) and more glue if a fault does pop up.