Power factor is a measurement of the energy
generated by a particular cartridge in a particular firearm. It is basically computed the same way that one computes momentum and that the projectile's velocity
are both directly proportional
to the resulting power factor.
The formula for power factor is:
bullet weight in grains * muzzle velocity in Feet per Second == power factor
IDPA takes the resulting number and uses that as the power factor but IPSC divides it by 1,000 since the last three digits are quite insignificant.
Measuring power factor is done by taking one's firearm and ammo and shooting three rounds through a chronograph at a distance of ten feet. To "Make Major" one's firearm and ammo combination must achieve 165,000 or 165 power factor, in IDPA or IPSC respectively.
Failure to meet these requirements will result in being classified under "Minor Factor" and your hits will mean less points. Failure to meet the minimum power factor of 125,000 will mean disqualification of competitor's equipment.
Note that power factor is just a measure of momentum and is in no way a definite measure of a round's stopping power for as we all know there are three important factors that constitute firearm stopping power, shot placement, shot placement and shot placement. A hit from a .22lr beats a miss with a 44 Magnum and of course a .22 in the hand beats a .45 at home, every time. Carry a gun!