After reading Baker Boys answer, I'm confused about whether you mean using P in a modern cartridge rifle, or P in place of RS or FF black in a BP muzzleloader...?? If you're using it in a cartridge rifle, the same rules apply as with black, the most important being that you leave no dead air space between the powder charge and the bullet.
P is a substitute for FFF black, so it would be similar to using FFF in place of FF. 3F is used in BP pistols & revolvers, while 2F is mainly used in rifles of .54 cal and up. You would get a little more "steam" behind the ball, resulting in a bit more velocity and pressure, but if the rifle is a modern BP gun, and you weren't exceeding the recommended charge of FF, you would be just fine. For instance, in an H&R Huntsman .58 cal, H&R recommends no more than 100 grains FF (volume) as a max load, but if you used 100 grains (volume) of P (pistol) instead of RS (rifle/shotgun), it isn't going to come apart.