Bullet movement begins when combustion pressure reaches approximately 2800 to 3700 psi depending upon the bullet retention characteristics of the cartridge and barrel. Bullet movement is caused by compressed propellant pushing on the bullet base in most rifle cartridges, That is, if the cartridge is long enough that the primer does not ignite all propellant in the case
At first bullet movement a shear line is set up in the propellant, which is approximately the diameter of the bullet, extending back to the burning propellant interface. This shear surface ignites much more rapidly than the normal propellant burn rate due to heating by infrared radiation. Shear lines in solid rockets have ignited as rapidly as 120 inches in two milliseconds at 500 to 1200 psi. Shear lines in gun cartridges would be expected to ignite even more rapidly in the pressure range of 3000 to 30,000 psi. As the shear line burns radially both inward and outward and the bullet accelerates into the barrel, maximum burning area is reached within the cartridge and peak pressure is generated. Propellant burning continues within the cartridge and barrel, further accelerating the bullet until it reaches the muzzle. Best efficiency occurs when the propellant accelerating down the barrel behind the bullet is minimized and propellant retained within the cartridge is maximized. Propellant trapped in the case burns at higher pressure whereas propellant moving down the barrel burns more slowly due to acceleration and lower surface pressure.
Tests performed by M. L. McPherson, published in "Metallic Cartridge Reloading, third edition published by DBI Books Inc, 1996, pages 45 through 49 give propellant compression for most