Did you compute for air in the barrel after calling out there was no air in space? Just curious about that, s’all.

With that, there would be a hell of a suction on the bullet after the cartridge was fired. Also, the detonation doesn’t happen all in one go and continues as the bullet moves to the muzzle. (I did quite a bit of experimenting with that to reduce muzzle flash, actually.) So, the bullet is accelerating until the pressure is released when the bullet passes the muzzle.

While air in the barrel isn’t really a factor on earth, surrounding air pressure absolutely is. It affects burn rate most but how it affects burn rate is a characteristic of the powder itself. (In zero-G, I would speculate a slower burn because the powder would be more prone to floating if it wasn’t a compressed load.)

When I plan to go to lower altitudes, I typically use lighter loads or I risk over-pressure conditions. (I’ll basically just compute for a couple hundred pounds less pressure chamber.)

After all that, I have no fucking clue what would happen in space because the conditions are so wonky.