Gear Rifles - design assistance requested

[Anthony]

Originally Posted by Varyon

Every doubling of barrel length would halve the average amount of pressure during the acceleration. That makes a longer barrel a zero-sum game - doubling barrel length doubles the distance over which the force operates, but halves the average force (with constant area, pressure simply becomes force), and given kinetic energy is equal to force times distance, that gives us constant KE. Considering friction and drag, that actually indicates a longer barrel results in less velocity. Am I missing something?

Yeah. Pressure isn't constant in the barrel, so you have the same pressure at 6" away from the bullet entry point regardless of final length. Length typically doesn't give you really large velocity increases, but it's non-zero. In principle there are methods of introducing gas from the sides of the barrel that result in substantially higher velocities, but hard to do without electronic switches.