In our last post, we looked at one way to study the performance of cartridges by calculating the average kinetic energy of the bullet. In this post, we will look at another method that some people use to calculate the effectiveness: momentum.
Back in physics class in school, some might have studied that momentum = mass * velocity. So all we need to do is use a scale to measure the mass of the bullet and use a chronograph to measure bullet velocity and we can calculate the momentum of a bullet.
The idea is that when a bullet hits the target, the collision will cause the momentum will be transferred to the target. Unlike the calculation for kinetic energy, which we studied in the previous post, momentum is proportional to the velocity, whereas kinetic energy is proportional to the square of the velocity.
Calculating the momentum of bullets for comparative purposes works when the bullet speeds and bullet weights are both moderate. This method completely ignores factors like bullet diameter, bullet shape, bullet material etc. Therefore some of the results could be a bit stilted. For instance, a 500 grain .458 Winchester magnum fired at 2000 feet/sec has slightly less momentum than a 3 pound bag of cotton moving at 50 feet/sec. However, no sane person will ever argue that the 3 pound bag of cotton will effectively stop a charging elephant or rhino!
On the other hand, if the bullets are made of similar material and of similar shape, then comparing their momentum values is not a bad idea to measure which one will be more effective than the other. Most of the authors that use momentum to compare bullet effectiveness tend to be fans of big-bore heavier bullets.