Kick The Habit (1) - A Tongue In Cheek But Serious Treatise On Recoil
Updated: Aug 13, 2020
Recoil Is What It Is - Live With It And Do Not Flinch Or Be A Jerk On The Trigger
The British Textbook of Small Arms, 1909 Edition touches on the dynamics behind the recoil impulse in a commendable effort to take the late Mr. Newton’s laws on the dynamics of mass in motion to its final conclusion. The author states: “The physiological sensation produced by recoil is generally termed the ‘kick’. It is probable that the kick is intensified by the rapidity of the first rise in velocity, which with the Lee Metford is attained in about .0006 seconds. The shorter the time in which this maximum is reached, and the greater the velocity, the more violent the kick. From this it follows that the kick, though no doubt depending chiefly on the energy of recoil, can not be said to be strictly proportional to the latter alone.. ”
“... the kick.. can not be said to be strictly proportional to (energy) alone”. Indeed so - but in fact kinetic energy has nothing to do with recoil at all. Some gun forum experts, even nowadays 111 years after 1909, still fail to consider what the first part of that quotation really signifies. A USA firearms litigation expert and author of one book on rifle design is hung up on the principle of momentum instead of kinetic energy being the cause of recoil force - which in principle is commendable as at least he understands that kinetic energy is not a force so it can not do any work - let alone kick the shooter. Then he affirms that at Newton’s Third Law the explanation of what recoil is ends right there. In one post addressed at me he eloquently stated that the opposite reaction to the bullet’s momentum as it leaves the muzzle causes the opposing and equal recoil force, because “Newton is Newton”. Newton’s Third Law of Motion is indeed at play - but not in the sense what the bulk of this gentleman’s argument avers to.
The recoil “kick” is not proportional or even related to kinetic energy at all - and neither is it the rifle opposite and equal reaction to the bullet's momentum as it exists thre muzzle. It is related to momentum, though. Momentum is not a force, but at least unlike kinetic energy it can be translated into becoming a force. Kinetic energy can only be translated into becoming heat. Acceleration of the bullet and deceleration of the butt pad against the shooter's shoulder within a certain time frame determine the force of recoil because Force = Mass x Acceleration. Recoil starts the moment the bullet starts accelerating out of the case mouth. As acceleration of the bullet increases, the recoil force increases.
In various forums of the USA gun media there are shooters who seem to be obsessed by recoil and how punishing it is - even the .270W is regularly found guilty as charged of this sin against shooters. While the .270W is a totally adequate little rifle to hunt non-dangerous game if bullets of the proper design and construction and weight are employed it has no recoil to speak of. Right at the end of this series there is a table of the raw recoil force of different cartridges in representative rifle weights.
Many good men aim to save money by reloading their own ammunition - and then spend their money on reloading equipment, and chrono logistics, and accuracy gadgets, and gallons of propellants of all makes and burn rates, and bullets, and brass cases. They invariably waste a life of time and resources by shooting while crouched over a bench instead of doing some real hunting. By buying and shooting many, and poorly designed and poorly shaped cheap rifles instead of acquiring one (O.K. maybe three) best quality hunting rifles with chamberings that compliment and not duplicate one another. Rifle brands that are reliably designed to make it easy for the hunter to kill his animals, and for his generations to come after him.
Instead of owning three excellent quality rifles of complimenting chamberings and sticking to a single type bullet of one weight on the heavy side for each, thousands and thousands of little holes are studiously created through sheets of pulped dead trees from a safe 100 yards away. Scores of brands and types and weights of bullets are being “tested” this way, with iterations of propellants. With most reloaders a final conclusion to settle on the best combination for future hunting is never reached.
This means shooters are forever stooping unnaturally into a rifle (“crowding it”) at a shooting bench - and then complain about recoil - and endlessly obsess and analyse, and quantify and speculate; and often jump to confusions. Very few men shoot their rifles as was required by the very design philosophy of the “long arm” - meaning standing up and shooting it off-hand from the shoulder. The design of form and fit of the hunting rifle is NOT suitable for stooping into it over a low bench. Shooters try and overthink the reasons why they feel pain from “recoil” when in fact there is very little recoil. It is all in the shooter's attitude - physically, as well as psychologically speaking.
The proper way to hold a rifle:
Very few men shoot their rifles as was required by the very design philosophy of the “long arm” - meaning standing up and shooting it off-hand from the shoulder. The design of form and fit of the hunting rifle is NOT suitable for stooping into it over a low bench. That form of holding it does not follow function.
Not the best way to hold a rifle for a naturally positioned and relaxed wrist, steadiness, and recoil absorbtion by the arms. Recoils starts before the bullet leaves the muzzle and will immediately slightly yaw the upper body to the right because of the assymetric geometry of the arms-rifle system.
There is a grudging acknowledgement that hunters - as a specie - seem to not appear to know about this kicking conspiracy against the shooting community - maybe for reasons of some hidden psychological challenge they apparently must suffer from. Almost like military vets returning from Vietnam / Afghanistan / Iraq - every shooter, compelled by some unnamed force conspired by the Very Affairs of the veteran shooters’ media, is expected to automatically suffer from some disorder - in this case a fear of perceived recoil. To be accepted in the eyes of his brotherhood he is expected to develop and display PTRD syndrome (Pre Traumatic Recoil Disorder). That, while in the field “I did not hear the bang or feel the recoil”.
As an objective observer and hunters’ guide I regularly experience that visiting hunters may say they felt no recoil at the shot. Results often show they still fear it, anticipated it and flinched in anticipation of that recoil force, or jerked the trigger so that the animal’s heart was missed with the shot.
PTSD is not the to be expected outcome of doing a normal manly thing like shooting and killing a quarry for a valid, ethical purpose - like self preservation or national preservation. Neither is PTRD - and neither needs it be when shooting at an already dead paper target. It is not even a womanly thing, because there are quite a number of slender ladies who, on foot, guide visiting hunters and / or local hiking enthusiasts around South Africa’s Big Six country, carrying a .416 Rigby or .375 Holland & Holland, AND shoot it regularly and accurately without flinching. All in order to maintain her PH or guide registration, but more importantly to be able to shoot and kill that sudden charging elephant or Cape buffalo or a lioness - either of which one day WILL happen. These psychological aspects of recoil will be attended to in a follow up blog post - but first the fascinating facts of physics.
In a further statement on this subject the British Textbook of Small Arms alludes to the baseline principle from which this discussion shall proceed, namely mass in motion:
“As regards the sensation [not “sensational aspect”] of recoil, it seems well established that the actual velocity of the [rifle in] recoil is a very great factor. In shotguns weighing six to seven pounds, fifteen f.s [feet per second recoil velocity of the gun] has been long established as a maximum - above which gun headache is sure to ensue. With an elephant rifle perhaps weighing fifteen pounds [the 4-bore of Courtenay Selous] such a velocity is unbearable for more than two shots”.
The premises for the discussion to follow are the following:
1. Recoil is a force that has an impulse on the shooter’s shoulder, or letting his thumb make an impulse against his nose or cheek bone.
2. An impulse is nothing but “g” (acceleration or deceleration) that is opposed by another object that the acceleration or deceleration acts upon. “g” also is the symbol for gravity. Gravity is the field that attracts every thing that has mass towards the centre of the earth, and continuously accelerates it in free fall. When a parachutist whose parachute had failed to deploy hits the ground he experiences a certain “g” force depending on his velocity at the time, and the rapidity by which the ground decelerates his body from say 130 mph to zero. That time frame can be calculated depending on the relative softness or hardness of the ground. His deceleration over time is the basis for the recoil impulse exerted by the ground on his hapless body. In itself acceleration or deceleration is not a force - only when it acts upon another body and receives the equal and opposite counter impulse.
3. In the case of hunting, the reaction against the animal by the deceleration of the bullet during its short time of impact and penetration is an impulse, the quantum of which, when spread over the bullet frontal area determines the bullet’s penetration ability and the mechanically inflicted trauma the animal’s heart or brain will experience. In the case of recoil impulse under discussion, the reactive deceleration by the rifle (from whatever start velocity to zero) against the shooter’s shoulder while the bullet accelerates through the bore under the force of gas pressure, also is an impulse - the quantum and time of application of which, when spread over the surface area of the butt pad determines the discomfort the shooter will experience.
4. For something to be accelerated or decelerated by or onto an external body it must have mass.
5. That same something must at some stage possess a “velocity” - in our case the velocity at which the bullet leaves the rifle’s muzzle, or the velocity of the rifle when it meets the shooters shoulder.
6. There must be a time frame for the acceleration or deceleration to take place.
7. Impulse is the force applied during this time of deceleration of the body to the object that causes the deceleration - in our case the shooter’s shoulder.
Calculation of recoil
Generally speaking, during any moment after the trigger has been pulled there develops an increasing and then decreasing force of gas pressure that acts onto the base of the bullet, forcing and accelerating it towards the muzzle, and at the same time that same force of gas pressure is exerted onto the rifle to the same value, accelerating it rearwards onto the shooter’s shoulder. From after having pulled the trigger, if at any instance we could freeze the thermodynamic actions inside the bore we could observe that the total pressure force onto the base of the bullet will be exactly the same force pushing back onto the rifle. With internal ballistics’ software we can see and break this view up into tiny fractions of a thousandth of a second.
Had the rifle weighed the same as the bullet the recoil velocity of the rifle would have been the same at any instant as the velocity of the bullet during the bullet’s time in the bore because Force = Mass x Acceleration. The mass of a CZ 550 .416 Rigby with four cartridges is the same as 162 of those shiny 400 gr Peregrine bullets, so the rifle’s recoil acceleration is about one hundred and sixty times lower than that of the bullet. The reader immediately sees the reason that the heavier the rifle, the lower will be the recoil force.
In reality the rearward acceleration of the rifle due to only the forward acceleration of the bullet is smaller than the full total rearward acceleration of the rifle in practice, meaning actual recoil is more than what is caused by the bullet’s passage. This is so because there are two more forces adding to the recoil, namely, 1) the rearward reaction by the rifle to the forward acceleration through the bore of the mass of gas of the burning propellant charge, and 2) the rearward reaction by the rifle due to the outburst of the propellant gas after the bullet had left the muzzle. This high velocity expulsion of gas under very high pressure acts like the thrust from a rocket engine and slams the rifle back AFTER the bullet has left the muzzle, adding to the other two recoil forces. (This mass of gas has interesting effects on the bullet which it envelops and overtakes outside the muzzle - but that is a discussion for another time).
The recoil forces due to acceleration of the bullet and gas can easily be calculated and when applied to the surface area of the butt pad against the shooter’s shoulder a “felt” recoil impact impulse can be determined.This impact impulse is still subjective for individual shooters, depending how much of his own weight is added to the rifle (his arms), and how he is holding it, and whether he is standing upright and riding the recoil or stooped over the butt at a shooting bench, or lying prone behind the rifle.
.416 Rigby cartridges with 400 gr Peregrine VRG-2 flat nose monolithic copper solids
Recoil due to the bullet’s acceleration. Force = Mass x Acceleration, so the 400 gr bullet from our .416 Rigby in a 660 mm barrel is accelerated from zero to 2,425 ft/sec (let us stay with the metric values, namely 740 meter/sec),within a time of 1.4577 millisecond.That relates to an average acceleration rate of 760 meter per second every second. Put another way, that is a force of400 gr x 760 Newton.Second (Ns).To express all the variables in metric units the equation looks like this: ((400 divided by 7000) x .454) x 760. So impulse force by the bullet’s acceleration causes an equal and opposite recoil force of 19.7 Ns.
Recoil due to gas acceleration. 77 gr of propellant was converted to the same mass of gas which also was accelerated at average 760 m/sec squared, and this adds 1.84 Ns to the recoil impulse, increasing the recoil force to 21.54 Ns.
Recoil due to gas back-thrust at expulsion. This is a more involved calculation so I let my software do that, and this force adds another 3.8 Ns impulse to the reoil force, bringing the total raw recoil force to 25,34 Ns.
Effective recoil force on the rifle. 25,34 Ns accelerated the 400 gr bullet to 740 meter/second. The rifle is 162 times as heavy as the bullet. F=Mass x Acceleration, so the force of the rifle onto the shooter’s shoulder is .16 Newton per second. BUT the shooter’s shoulder experiences that within a mere 1.4577 millisecond, so the actual recoil impact impulse on his shoulder by the .416 Rigby is 108 Newton.
That is quite a figure, but it is still manageable by any 16 year old boy if he shoots the rifle off-hand, holding it properly, and rides the recoil. That he can do a number of times a day and will only show some green/blue broken blood capillaries under the skin of his shoulder that evening, but nothing painful when he pulled the trigger.
The 108 Newton recoil of this rifle can be handled by any man. It is a very pleasing rifle to shoot and with the Peregrine VRG-2 bullets a most capable performer on Cape buffalo, elephant and hippo. BTW there were three crocodiles in the photo against the opposite bank of the Maxaxese River I had just crossed on foot downstream to the right.
The .270W in comparison: Consider an 8 lb .270 Winchester shooting a 130 gr bullet at 3,100 ft/sec (915 m/s). Total recoil impulse is 12.93 Ns. The rifle weighs 421 times as much as the bullet. F=mass x acceleration, so the impulse onto the shooter’s shoulder is .03 Newton per second. But he experiences that impact within 1.1 millisecond, so the actual recoil impact force on his shoulder from the .270W is 27 Newton. That is only 25% of the recoil of a .416 Rigby or Ruger.
Is physics not a wonderful tool? It removes all the circling assumptions expressed around forums. It says “Kick the habit of worrying about and flinching in anticipation of recoil”.