Barrel pressure ahead of the bullet

[chromatica]

There is a volume of air in the bore ahead of the bullet that has to be accelerated from rest to the final muzzle velocity. If you can figure out how long the bullet remains in the barrel, you could figure out the average force required to do this.

 

[chromatica]

There is a volume of air in the bore ahead of the bullet that has to be accelerated from rest to the final muzzle velocity. If you can figure out how long the bullet remains in the barrel, you could figure out the average force required to do this.

Because most rifles are launching bullets at Mach 2.0 and above, there is not enough time for the pressure to equilibrate to atmospheric pressure. The whole plug of air will pretty much blow out the muzzle as the nose of the bullet reaches the muzzle.