The formula for finding the acceleration of a bullet is A = v^2/2l, or velocity squared divided by two times the barrel length. Velocity means muzzle velocity, or the speed at which a bullet leaves the barrel of the gun.
If a bullet leaves the muzzle of a rifle at 600 meters per second, and the barrel is .9 meters long, the acceleration within the gun barrel is 600^2 divided by 1.8, or 2.0 x 10^5 meters per second^2. Bullets are propelled by the rapid expansion of gas within the gun barrel. When the bullet leaves the barrel, it travels at its maximum velocity and no further acceleration occurs.