like a spring piston...

[Aaron ?????]

ive come up with an idea... its like a spring piston gun exept instead of a spring at the back of the piston is conected to another 2" piston with a solenoid valve and tank behind it...
in a normal air rifle the piston only has to move at 41 fps to get the pellet going at speeds of around 1000 fps
so if we could propel a piston at 100 fps (which wouldnt be that hard using 100 psi of air on a 2" diameter piston) we should be able to reach mach 2!!!
its a bit hard to explain but someone who knows a bit about air rifles should know what im trying to explain

[Animal81182]

sounds good but what do you intend to use for ammo - as this will effect the fps and velocity

a small shaped slug would work really well with the weight in the front end i bet

so are you planing on having the firing mechanism similar to an air rifle - meaning using your piston to compress the air as it moves behind the ammo - and once reached your thresh hold, force your ammo down the barrel and out at a high fps???

Animal http://UKSpudGun.com

[Ballad_Black]

Sounds crazy man, a Mach 2 tater or other ammo. Lol, you'd have to have some thick@$$ PVC I'd guess, or maybe 100+FPS just sounds more than is.

[boilingleadbath]

You are going to run into issues with the speed of sound, just like every one else. This will be slightly alleviated by the compression heating, but not too much.

[Mike32]

ya ive gone over this alot as well, you need hundreds or thousands of psi to get shock heating that substantially will effect the airs speed of sound. Otherwise you could heat the air or use Helium as your gas.
sucks dont it

[Aaron ?????]

the idea does sound possible with the gamo hunter 1250 air rifle the spring has around 100lbs of force behind it and that gets up to 1250fps so with a 2" diameter piston at 100 psi i could get over 400lbs of force behind it!!! but i probably will get problems with the sound barrier but im pretty sure i could break it.
if i get a lathe in the future ill try this out ithink ill use steel pipe as the main compression cylinder and sch 80 as the 2" cylinder and animal81182 ill be using .177 cal air rifle pellets in this gun if i used potatoes they would probably end up as steam out the and of the barrel

[boilingleadbath]

Well, now, that gives me an idea - if the piston was somewhat massive, you could use a small pneumatic launcher powered device to provide you with high pressure air, based on the princiable of the 2 stage light gas gun. With proper design, pressures in the thousands might be possible.

[Mini Khan]

sorry but that wouldnt work because the piston compresses air to shoot the pellet, and if u have air behined the piston at a 100 psi and relise it then the priston wont compress ,more than a 100 psi on the other side, so u might as well just forget the piston.

[Brian the brain]

The idea is to connect a large ( primary)piston to a smaller ( second)one connected by a rod.

[boilingleadbath]

"The idea is to connect a large piston to a small one" -brian
No, acctualy it's not.

A 2 stage gas gun is not a static system, and you cannot examain it satisfactory as one - it is neccesary to examain it as a dynamic system.

First you accelerate the piston in the pump tube to high velocity with a typical pneumatic system.
Appon nearing the end of the pump tube, the piston starts slowing down, as the pump tube pressure starts to increase. However, because it's been accelerating for 85+% of the pump tube, it takes a LOT more force to stop it in the last 15-%... an average force in the last 15% of ~700 psi, which means that the max pressure is indeed higher than that, probably well above 1,000 psi.

And that's not even an optimized system....

[Brian the brain]

So it's the momentum of the piston that creates a higher pressure on the other side??

[kenbo0422]

Simple pressure problem:

100psi on 1" dia. piston = 100 lb/.78539 in^2 = 127lbf

Piston is connected directly to a 1/4" piston (like a 'T' shape looking at it from the side)

So: 127lbf on 1/4" dia piston will yield: 127lb/.04908in^2 = 2587.6 psi max

The pressure goes up with 1/8" to 10,350.44 psi (about the same as a small rifle.

Using a pressure relief setup, a spring loaded piston that cracks open at say 3000 psi, you'll have a nice bang for your buck. Go to McMaster and get a nice strong spring to pressure a 1/4" piston shut until 3000psi is reached. Try it. but use steel fittings for tubing.

[ALIHISGREAT]

talk about kicking up an old topic LOL

[kenbo0422]

Yeah, I'm new here, although my habits have been shooting regular guns. I've been kicking around some ideas to make a low cost gun and ammo from the usual hardware/McMaster type stuff. So far it looks promising. With what I'm looking at, a hunting rifle could be made for next to nothing. Even firing a potato at something is pretty lethal, but I don't think they let you through the pearly gates for death by potato...

[Sticky_Tape]

Surprizing nobody tried this yet.