Updated: Feb 19, 2020
Hunters have - and apparently will forever discuss the dynamics behind bullet penetration into an animal at the campfire or on internet forums. Some confident affirmations or armchair theorising are often posted and also proposed by gun writers in shooting magazines. More often than not the motion laws of the late Isaac Newton are invoked by some (mostly not by gun writers because on a general knowledge level about hunting big game they are up there with youthful salesmen behind the counter in the average gunshop). More often than not the late great phycisist is quoted in either a simplistic or incomplete manner.
Penetration of a bullet into whatever medium is dependent on the magnitude of a force. The equation to calculate the magnitude of a force needs values of two entities: mass and acceleration because a logical deduction of Newton’s Laws is that Force = Mass x Acceleration. This immediately will remind the reader of what he had learned in grade 6 science in school: Two raw values of a moving object namely kinetic energy and momentum can immediately be ignored as having any direct relevance to penetration as neither is a force.
Kinetic energy does not represent any force as it merely is a scalar reading on a joule scaled thermometer. Momentum, which indeed is a vector quantity and not a scalar reading like kinetic energy, also, by itself is not a force. So the raw momentum value of a traveling body has absolutely no value in predicting or calculating the force for it to penetrate anything.
Momentum can however be used as the pre-impact vector quantity to be integrated with negative acceleration (deceleration) of the bullet during impact to calculate the impulse force which is the force that determines one of two aspects of a bullet’s ability to penetrate whatever medium it is impacting.
When firing a rifle, the recoil impacting on the shooter’s shoulder is an impulse force and in principle it is quite comparable to the impulse force which causes a bullet to penetrate or not. In a 30-06 shooting a 180 bullet it is the opposite reaction onto the rifle of the mass of the bullet being accelerated through 24” of barrel from zero to 2,740 ft/sec within 1.20 milliseconds. The bullet undergoes a positive acceleration (speeding up) and the rifle experiences the negative value of that acceleration (slowing down and ending) as a recoil impulse on the shooter’s shoulder. In a future post the issues of recoil will be discussed in more detail - the reader may take note at this stage that the recoil force is “negative acceleration” as it is the “kick” force the shooter experiences. It is the impulse caused by the deceleration of the rifle and stock over the time due to the the bullet’s acceleration through he 24” of barrel. Remember this fact when the bullet’s penetration force into an animal’s body is considered below.
When on a bowling green a 2 kg bowling ball is rolled at a speed of 6 metres per second the momentum it possesses is 12 kg.meter per sec.
Similarly, a 286 gr bullet from a 9,3 x 62mm hunting rifle just before impacting the low shoulder of a Cape buffalo at 670 metres/sec also possesses 12 kg.metres per sec momentum. (the calculation is easy: (286gr ÷ 7,000 (the amount of grain in a pound) ÷ 2,2 (the number of pounds in a kilogram)) x 670 = 12 kg.m/sek. (metrication is beautiful, is it not?).
From experience we know that the 9,3 x 62 bullet with its 12 kg.m/sec momentum, if it is a flat nose VRG-2 Peregrine monolithic solid WILL penetrate the 1” thick skin of a Cape buffalo, WILL break the shoulder humerus bone and punch through a 1” thick rib and slash open the heart top chambers and punch through an opposite 1” rib and break into and get stuck in the opposite humerus-scapula shoulder joint. It is not able to break both shoulders and penetrate all the way through as will a .375 H&H similar design 300 gr bullet or a 400 gr .416 Rigby bullet or a 500 gr .458 Lott bullet. That is the very reason it is the minimum calibre and size cartridge allowed for Cape buffalo hunting in South Africa.
Should one throw that 2 kg. bowling ball at 6 metres per second also possessing 12 kg.m/sec momentum against the Cape buffalo’s shoulder it will be clear to even the firmest believer in the myth of the hunting value of momentum figures that it will not achieve the same penetration as does the bullet from the 9,3 x 62 with the same momentum value.
I am unsure what effect the bowling ball's 12 kg.m/sec momentum will have on the average Cape buffalo bull, but I am pretty sure what it will do to a buffalo cow possessing a bout of PMS, having run away from fool PMS possessed buffalo cows more than once in my life.
Momentum figures on their own, as much as kinetic energy figures, are useless to the hunter for projecting whatever effect it will have on an animal.
Why does the same momentum value of 12 kg.m/sec of the bowling ball not achieve the same effect on the Cape buffalo shoulder as does the 12 kg.m/sec of the 9,3 x 62 bullet?
1. The impulse force which is the impact momentum applied over time (the time of deceleration) is not the same, as the 2 kg. ball will have a much, much slower deceleration than the bullet and therefore a much lower impulse force, and
2. The frontal area onto which the impulse force is acting upon of the bowling ball is a great deal more (about 2,000 square mm compared to the 68 sq. mm frontal area of the 9,3mm bullet), so the impulse force per square millimetre of the bowling ball will be far less than that of the bullet. That is a measure of the frontal area profile drag opposing the impulse force of the projectile.
In the next post actual figures to calculate relative penetration indices for various chamberings and bullet designs will be discussed.
Some bullets and their penetration ability:
Very big Cape buffalo bull shot with a 480 gr Peregrine VRG-2 monolithic solid from a .458 Lott. The bullet went in one side and out the other and was gone. It severed the aorta group from the heart top chambers and the heart burst open from the inside from top to bottom. The bull fell in its tracks.
This 300 gr. PMP ProAmm (similar to the Hornady Interlock) from a .375 H&H penetrated a zebra in a virtual Kentucky heart shot all the way from the rear, demolished the tenderloins, ripped open the top of one lung lobe and stopped against the neck vertebra - that was more than 4 ft penetration. The zebra went 300 yards and we followed its tracks and a little meat wound blood now and then. The bullet retained 87% weight.
The 150 gr Peregrine VRG-3 from a 30-06 penetrated straight ahead all the way from the rear, broke through the warthog's back vertebrae behind the neck and on contacting the skin tumbled, the rear end penetrating the skin and the mushroom snagging on the inside.
Perfect, un-petalled mushroom of a 300 gr. Peregrine VRG-3 bullet from a .375 H&H. It penetrated the 1" thick shoulder skin of the Cape buffalo bull, broke the humerus bone, broke through a 1" thick overlapping rib, cut open the top chambers of the heart, broke an opposite 1" thick overlapping rib and bulged against the opposite skin.
The bull went 30 metres and was dead.