Gear Rifles - design assistance requested

[thrash]

Originally Posted by Varyon

I'm not sure a flechette would be possible, although perhaps if it's contained within a sphere of wax?

Or just a flat plate plug as a sabot. Wax would add weight and drag and thus slow down the round.

It also seems like a longer barrel wouldn't be as useful - for firearms a longer barrel means more of the powder burns before the bullet leaves, but this has all the pressure generated in an instant, going down as the bullet travels down the barrel.

Right -- but as long as the bullet is still plugging the barrel, it continues to get some residual acceleration from the pressure behind it even as that pressure drops. Ideally, I imagine you'd need the pressure to be above (1 atm + friction + drag at muzzle velocity) as the bullet exits the muzzle. That, in turn, should be a function of the ratio between volumes of air in the piston (before firing) and the barrel (after). Since both of these depend (in part) on the overall length, I suspect there's an optimal length for any given caliber.