SpudFiles

For my science project I am going to build a air cannon and fire it at the same pressure and angel but with different density gases ,and see if the projectile would go different distances. My original plan was to use helium, air, CO2 and Sulfur hexafluoride. I need help with two things, first, do you think using different density gases would make a difference. Second, pressurized tanks of these gases ,for less than... a lot of $,have been quite elusive. Any feed back about were I could get these gases or if it would even work would be greatly appreciated.

While I don't know where to obtain cheap tanks of these gasses, I can tell you that each one of these gasses will yeild a different muzzle velocity. This video by the MythBusters will explain it.
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EDIT- A tip for you... use a valve that is consistent, like a sprinkler valve, but not a ball valve. This will give you the most accurate results. You will also need an accurate tool to measure velocity, like a chronograph.

From a strictly theoretical perspective, yes, different gases will make a difference. The question becomes: Will you actually be able to measure/see that difference? The answer to that question is "It depends."

If you've a high performance gun that's pushing the speed of sound with choked flow in the throat? Yeah, you'll notice the difference. If your gun is of a lesser design? No, you won't notice the difference.

Pretty much what D_Hall said. At low pressure and a heavy projectile, you won't find much measurable difference with one exception.

At low pressure and low velocity projectiles, the amount of projectile leakage will make a difference. Light gas will blow past a poor seal providing less force on the projectile as the chamber pressure is rapidly depleted during the shot. Your heavier gases work better with poorly sealed projectiles.

This is why Paintball works well with CO2 and 300 FPS muzzle velocities.

On the other end of the spectrum as D_Hall mentioned, with highly efficient valves and light weight projectiles with a good seal, the transonic speeds can be higher with lighter gas.

So in conclusion, it depends.

thank you guys for all the information. And thank you Gun freak for mentioning a chronograph, as it will solve some problems. because now I can shoot though a chronograph in to a back stop rather than just shooting it and measuring the distance. the chronograph will eliminate some variables.

Try to obtain small containers of these gases from somewhere like weiler welding, airgas or praxair, these are all US companies, i have only dealt with weiler

Edit: Sulfur hexafluoride will be the only gas that will be dificult to obtain. Hell you could get helium(balloon tank) and co2(paintball) from walmart. Use compressed air from a compressor.

http://www.airgas.com/browse/productDet ... =SH%20CP80
you could try this but 80 cubic feet is alot of sulfur hexaflouride

I forgot to mention, the gas flow through the valve will be better with light gas if you are using a restrictive valve. You will see a large variable if using a 3/4 inch sprinkler valve to feed a tennis ball barrel. A lot more Helium can get through the restriction with less loss. By the same token a leaky projectile will spill much more gas.