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Old January 11, 2019, 01:43 PM   #7
Senior Member
Join Date: March 23, 2008
Location: Indiana
Posts: 697
I don't think this question has a simple answer.

The "power factor" that is used to determine classes in USPSA and IDPA competition is a measure of projectile momentum. It is based on a simple product of projectile mass and muzzle velocity (momentum=mv), specifically projectile mass in grains x muzzle velocity in feet per second divided by 1000.

Conservation of momentum is an important law of physics governing the interaction and movement of bodies. This law would suggest that the momentum imparted to the projectile by the expansion of gasses in the barrel will equal the momentum imparted to the slide.

But perceived recoil may depend on more than just slide momentum even when shooting various cartridges from the same handgun. The velocity imparted to the slide might significantly affect the perceived recoil. Projectiles that leave the barrel more slowly might therefore result in somewhat slower slide velocity.

Here is a somewhat interesting article which attempted to control for barrel length and pistol weight, measuring magnitude of recoil based on the extent of pistol movement when it was fired clamped into a Ransom rest:

The authors also attempted to control for different powders and powder charges used in different commercial loads by firing hand loaded cartridges that had been charged with the same powder.

They then adjusted the hand loads to match the same power factor comparing .45 ACP to .40 S&W and 9 mm Luger to 38 Super.

They found that when the power factor was matched the recoil, as measured by Ransom rest pistol movement was virtually identical for .45 ACP compared to .40 S&W, and for 9mm Luger compared to 38 Super. This makes sense to me since they controlled for pistol weight. Therefore the momentum imparted by the powder charge to the projectile (power factor) would equal the momentum imparted to the pistol which would result in equal movement of the pistol within the rest, given the same pistol weight.

The measured recoil of .45 ACP and .40 S&W was quite similar even when powder charge and power factor were not adjusted for. But as many who shoot both calibers would say, .45 ACP recoil is often perceived as a slower "push" whereas .40 S&W recoil is perceived as a sharper "snap". The authors opined that the faster .40 S&W projectile which dwells in the barrel a shorter time than the .45 ACP projectile, produces a faster recoil impulse. This makes sense to me.

If you have an accurate measure of muzzle velocity, projectile momentum is easy to calculate for any given projectile mass. It is momentum in lb-ft/sec equals projectile mass in grains times muzzle velocity in feet per second divided by 7000. If you compute the projectile momentum for similar 9 mm Luger loads (FMJ from the same maker, for example) for 115, 124, and 147 grain projectiles, you will usually find that the momentum increases by 0.8-1.0 lb-ft/sec as you step up each projectile weight. That would imply that the recoil will increase with increasing projectile mass. But those heavier projectiles also have lower velocity and dwell in the barrel longer, so the velocity imparted to the slide might be less.

For what it is worth I have heard some claim that 115 grain produces more recoil than 124 grain when shot from the same gun, and some the opposite. And I have heard some say the 147 grain shoots softer than 124 grain, and some the opposite.
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