Spudtyrrant, please, if you don't know something, it's not usually a good idea to make it up.
spudtyrrant wrote:supersonic could be very possible at 400psi although your barrel would need to be at least a good 5-6 ft long.
No, like I said above,
not with air.
Just like having 4 people pushing a car won't get it to 100 mph any more than having one person push it, 400 psi of air will not get a projectile supersonic any more than 100 psi. That is to say, it won't go supersonic.
There are VERY exceptional circumstances under which it can happen. I doubt c11man has considered any of them.
also if you really want to get speed: smaller diameters + longer barrels = more velocity not the other way around
As far as sabots are concerned, you can achieve a greater energy in the saboted projectile with a shorter, wider barrel.
As long as the increase in barrel diameter in combination with the sabot's density decreases the overall sectional density in the barrel, this technique is perfectly workable for achieving greater velocities in a shorter barrel.
After all, why do you think the military uses discarding sabot kinetic penetrators (fired at around 1,600 m/s, about twice as fast as a typical rifle bullet), unless the use of a sabot could allow them to do exactly that?
c02 which is even lighter than air capable of prepelling a projectile over the speed of sound
CO2 is approximately 50%
MORE dense than air. It's speed of sound is much lower, and it's potential maximum velocity is closer to 800 fps.
Helium or Hydrogen would help, and would almost certainly guarantee supersonic results, but they're a little hard (read: expensive) to get hold of in meaningful quantities.
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