Absolutely what I was thinking. Except I'm using the ideas of impulse instead of energy. Kinetic energy is of course the best way to look at it when entering the target I would imagine. It's force X time which is impulse. So f and t both get bigger however, the force of the gas applied to the projectile must diminish as the barrel lengthens (unless the burn some how increases through time thus possibly amping up gas pressure)But you still increase the impulse (FXt) so the exit velocity should theoretically get bigger. But there comes a point of diminishing returns with barrel length. And then add in you are pushing a comparatively small mass when compared to a long distance hunting rifle for bigger game. So you get a large f on a small m and F/m = big ass acceleration (while in the barrel, after that you are done with gas pressure)
I do not know what an FXt graph would look like for this weapon with different length barrels but it would be very interesting. Your numbers up there are most likely averages and not force applied thru time which would be very difficult to keep constant. But it gives ballpark numbers.
Also air friction is going to most likely inhibit the smaller mass more but this is also highly shape dependent so again difficult.
Sorry for going on but this stuff fascinates me even though I don't own guns.
Oh, one correction. It's gotta be ft X lbs if you are talking energy, not ft/lb. / being divided by.