To add on to what LA says, in theory the pressure should go up a bit, although another factor is that a bit more "blow by" around the bullet happens until it fully seats into the barrel due to the somewhat higher pressure. My GRT tool says Pressure/MV increases but there's not a lot of MV change. When I compute my OBT nodes I generally assume coal changes only affect the timing of the bullet exit while MV is assumed constant. It's a simplification but seems to work ok.
The other factor as I know it, is that for a given charge of powder, there is a finite and fairly constant amount of energy to do the work of accelerating the bullet, so again, MV shouldn't vary by much.
But, as LA said, at the extremes of too short/too long-into lands, the pressure can skyrocket and do weird and dangerous things. Not a good spot to be in. Stay away from extreme Coal's.
The other factor as I know it, is that for a given charge of powder, there is a finite and fairly constant amount of energy to do the work of accelerating the bullet, so again, MV shouldn't vary by much.
But, as LA said, at the extremes of too short/too long-into lands, the pressure can skyrocket and do weird and dangerous things. Not a good spot to be in. Stay away from extreme Coal's.