Understanding Recoil
With a given fixed situation, i.e the combination of gun weight and design, caliber and charge, body mass, consistent technique etc, there is little the shooter can do about recoil. However; an understanding of recoil and how it affects the shot can be beneficial both towards technique and equipment selection.
The Basic Principles
| Recoil |
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 The blur of movement caused by the recoil of this 357 magnum is clearly visible. © Walter Loevens |
Recoil is governed by Newton’s third law of law of reciprocal actions:
- To every action there is always opposed an equal reaction:
- or the mutual actions of two bodies upon each other are always equal,
- and directed to contrary parts.
Or Simplified:
- ‘Every action has an equal and opposite reaction’
In other words when the gun fires, the bullet moves forward and the gun moves backwards with the same amount of momentum. Momentum is defined as mass times velocity. In momentum calculations, you can think of mass as weight.
So (ignoring the propellant gasses) a 2.6 gram 22LR projectile that leaves the barrel at 320 m/s would have a momentum of:
- (2.6/1000)*320 = 0.830
In accordance with Newton’s law the gun and supporting hand will have the same momentum in the opposite direction, so taking an average standard pistol with a weight of 1200g we can see that the pistol moves backwards at a little over half a meter per second:
- 0.830/(1200/1000) = 0.6916 m/s
Note: in the above equations, weight has been converted to the standard unit of kilograms, so the momentum is measured in units of kilogram-meters per second.
The energy of a bullet is normally calculated in Joules (metric) or foot-pounds (imperial). For formulas to calculate bullet energy see: Bullet Energy Calculations. Energy and momentum are different quantities, but are similar because both are determined by mass and velocity. An object’s energy is defined as one half of its mass times the square of its speed.
Although the above momentum formula is simplistic and does not take into account all the factors involved, it does provide us an overview of the main factors involved, and illustrates the relative momentums and velocities. If a gun was placed suspended and free to move, it would indeed move rearwards at the speed calculated above. When shooting though, the gun is not suspended or free to move; Once placed in the hand, or in the case of a rifle, against the shoulder, the effective weight of the gun increases many times, and although we “feel” the momentum being transferred, the actual rearward velocity is considerably less.
Two Recoils!
| Secondary Recoil |
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 Exaust gasses created by a 357 magnum. © Walter Loevens |
There are actually two distinct recoils from a gun;
Primary Recoil
This is the recoil, generated in accordance with Newton’s Third Law that results in the bullet getting up to speed before it leaves the barrel as demonstrated by the formula above.
Secondary Recoil
Secondary recoil occurs as/after the bullet leaves the barrel and is cause by the hot expanding gases behind the bullet leaving the barrel causing a ‘rocket’ like effect. Although also governed by the laws of Newton the formula for Thrust Calculation is not a simple one. Two things worthy of note are the duration and the masses involved. The duration is considerable longer than primary recoil. Primary recoil lasts whilst the bullet is accelerating in the barrel. Secondary recoil lasts considerably longer, from the moment the bullet leaves the barrel until all the energy of the expanding gasses has been exhausted. Secondly the masses involved are greater that the mass of the expanding gasses alone. The expanding gas displaces the comparatively low pressure surrounding atmosphere, in effect increasing the net mass involved to significantly more that of the expanding gas alone.
Managing Recoil
| Recoil Damping |
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 Pardini SP1 recoil damping springs and counter balance weights © Pardini |
Very little if anything can be done to reduce primary recoil, doing so would reduce bullet velocity; however, we can control the effects. Because of the vectors from the mechanics of the body some of the energy dissipates not backwards rotationally and upwards. The use of perforated barrels can help reduce the “rise” so the attempt to maintain sight alignment until the bullet has left the barrel.
For most firearms the effect secondary recoil is greater than the primary, fortunately it does not affect the bullet path since that has left the barrel. In gas operated semi-automatic firearms some of this energy is use up in recycling the gun. Further damping methods involving counter balances and springs are available in some designs. Lastly the incorporation of a muzzle break or compensators can dissipate gasses sideways and/or upwards, countering some of the gun’s movement.
The most significant effect on the shooter of recoil is sight alignment. This is particularly the case for Rapid Fire Pistol and the 10-seconds series of Standard Pistol when the sights must be re-acquired for the next shot. Prior the Athens 20004 Olympic Games the use of perforated barrels and compensators and .22 short ammunition were permitted for Rapid Fire Pistol event. Contrary to popular belief; recoil control and the ability to re-aquire the sights for a second shot are the primary reason for the two handed shooting techniques used in “Combat” shooting disciplines.
Today the use of perforated barrels and compensators is now restricted to only two ISSF disciplines; Air Pistol and 50M ( Free Pistol) and all Pistol and the use .22 short is discontinued.
The ‘good news’ is that your body cannot react to the recoil by the time the projectile has left the barrel – your reflexes are not that fast. ‘Flinch’ is in your anticipation of a shot, not a result of the shot!
