I wonder what difference that would make for a faster cartridge like a 5.56 which gets close to 3300fps in our atmosphere (depending on barrel length) as well as lower velocity cartridges like a .45ACP.

Would definitely create more heat issues with higher velocities too. Might need to go with water cooled barrels...

Would a longer barrel be LESS advantageous since there's less friction from air, the bullet's only friction would come from the barrel itself... would probably have to rework optimal barrel lengths for just about every cartridge...

It would probably be wise to switch to lower velocity, heavier rounds in space to offset those factors.

What about grease instead of oil for lubrication? Does grease evaporate too? Or maybe it evaporates slower? Or maybe it leave a film that creates reliability issues? Don't know...