GS CUSTOM BULLETS - Momentum v. Energy

All graphs and ballistic traces and tables on this site were calculated with Oehler Ballistic Explorer software.
GS Custom Bullets use only Oehler Chronographs for velocity measurement.

Momentum is bullet weight multiplied by bullet velocity. It is probably one of the most important factors to consider when comparing the potential terminal ballistics of different calibres and the different bullets available for a given calibre. It is not the same as energy which is half of bullet weight multiplied by velocity squared. One should also not make the mistake of looking at momentum calculated by using the unfired bullet weight and muzzle velocity either.

Often entire discussions will centre around the energy developed by a particular cartridge compared to another. Talk often goes around energy transfer to the animal and how this energy transfer kills the animal. Theories that the perfect bullet should stop under the skin on the far side in order to transfer all its energy to the animal is fundamentally flawed. Whenever comparisons are made, it is always a good method to take examples to extremes. That is where differences are the most graphic. To thus illustrate the flaw of energy and energy transfer, consider this: We take a 30-06 and shoot a kudu at 100 metres on the shoulders. In the first instance the bullet used is a 150 grain conventional jacketed lead match hollow point at 2920 fps. In all probability this shot will fail miserably because the bullet will shatter when it encounters the shoulder and not penetrate the chest cavity. It has an energy level of 2240 ft/lb at 100 metres and all of this is expended on the animal. The animal disappears over the horizon. In the second instance we use a 220 grain bonded core bullet at 2410 fps. The probability here is that the bullet will break both shoulders and exit. It has an energy level of 2155 ft/lb at 100 metres and a lot of that is “wasted” in the air behind the animal. This kudu however, goes nowhere but down.

This example also perfectly illustrates the importance of sufficient retained weight and, therefore retained momentum. The more a bullet breaks up, the more momentum is lost. A bullet that shatters completely, loses all momentum. Momentum is what carries the 220 grain bullet through the animal. The two bullets used had energy levels within 5% of each other in favour of the light bullet. The momentum figures are quite different. The 150 grain bullet had 55.6 lb-f/s momentum at 100 metres and the 220 grainer had 66, a difference of 15% in favour of the heavy bullet. Still this is an imperfect example and there is a better way.

This leads us to the next logical step. The match grade hollow point bullet broke up on impact, losing all momentum. The 220 grain bullet of more robust construction and subject to less violent forces at the lower velocity, suffered less breakup. The retained weight after impact, multiplied by the retained velocity at 100 metres, is an indication of the momentum level that carried the bullet through. From this we learn that muzzle energy and muzzle velocity are figures of academic interest alone. Impact velocity and retained weight are the figures we need to more reliably predict the terminal performance of a bullet. For want of a better term, lets call this requirement Terminal Momentum. The graph below reflects the real situation described with our two extreme example bullets where the 150 grain bullet broke completely and the 220 grain bullet broke to 180 grains.

Frequently the argument is advanced that the bullet should not exit the animal. This is only desirable where shots are taken in a herd situation. In every other instance an exit hole is preferred. Exit holes bleed more than entrance holes and are far more traumatic. On average, animals with exit holes are less likely to be lost than those with entrance holes only. We also see that bullet breakup inserts an unpredictability factor into our equations that is most inconvenient. If we want a bullet that performs optimally as far as maximum applied momentum is concerned, a couple of factors must be present. Firstly, on impact and penetration, the bullet must retain sufficient weight and it must deform to an efficient form that will create a double to triple diameter permanent wound channel. Secondly, it must have the highest possible ballistic coefficient and as much muzzle velocity as possible, in order to deliver its weight at the highest possible impact speed. The higher the impact speed, the higher the momentum values. It is also easy to fall into the trap of “heavy bullets are always better than light ones”. Remember that momentum has two elements: Velocity and weight. To illustrate the fact that lighter bullets can have more momentum than heavier ones, look at the following graph.

As we have seen, the lighter bullets that are driven faster, tend to fail more frequently. If one uses conventional jacketed bullets, and desire the reliability of good weight retention and the low meat damage that comes with the lack of fragmentation, slower and heavier is the way to go. This inevitably leads to two different requirements for bush and plains game if one uses only one rifle. If two rifles are available there is no problem. Simply set one up for bush and the other for the wide open spaces and hope that you are not required to take a 250 metre shot with your bush rifle. If you bump into a record book antelope at point blank range with your long shot rifle, you can always shoo it away a little. Life and hunting has never been simple or easy.

Ridiculous examples aside, who says we cannot have a one bullet load for bush and plains? It would certainly simplify one’s life. The answer to this question is Terminal Momentum. If we can equal the terminal momentum of the heavy bullet with a lighter faster bullet, and have that lighter faster bullet perform as reliably as the slow heavy bullet as far as weight retention is concerned, that would be very good. If that same lighter faster bullet is light and fast enough to run with our plains game load, it would mean one load for bush and plains. Consider the three way comparison graph below comparing a 150 grain High Velocity GS Custom bullet with two typical bush loads and utilising the Terminal Momentum principle. The HV bullet meets the requirements despite the light starting weight.

The requirement for a plains bullet is predictable terminal performance, but to that we must add the flattest possible trajectory, good resistance to wind drift, and low meat damage. Flat trajectory and reduced wind drift are the result of two factors: Good ballistic coefficient and high velocity. The bigger the numbers, the better. Contrary to some opinion, weight has nothing to do with either. Check the tables in any good reloading manual if you don’t believe this. The three graphs below reflect the trajectory, wind drift and momentum of two typical factory plains game loads compared to a 150 grain GS Custom HV bullet.

The majority of GS Custom HV bullets have extremely slender profiles with gently rounded boattails that result in high ballistic coefficients. So there it is. Develop one load for your medium calibre hunting rifle with the right bullet, and take it hunting in the bush or the wide open spaces and be ready for any shot that may present itself. The design goal with all our bullets is to simplify and improve your hunting experience.

To your success,

GS Custom Bullets, situated in Port Elizabeth on the East Coast of South Africa, manufactures solid copper, turned, monolithic bullets for hunting and sport shooting. These bullets are used by hunters on several continents, hunting from the smallest of antelope to the largest of dangerous game, using the smooth HP bullet, as well as the more popular HV, FN and SP bullets with the patented drive band concept. GSC bullets are configured for the highest possible ballistic coefficients. SP bullets are mainly used for sport shooting. All GS Custom Bullets are moly coated.