One of the most discussed - and least understood - subjects is recoil. To get the complete story we enlisted the help of Dr. Robert Birch, an expert on the subject at the Impact Research Centre of Liverpool University's Department of Engineering

What is recoil and recoil reaction?
Most shooters understand that recoil means to be pushed backwards by the shotgun or rifle during firing. Sometimes the meaning of recoil is misunderstood; so let's agree that guns recoil and we react to it. The kick that we feel, is in fact a recoil reaction. Recoil and recoil reaction are related but, they are not the same thing; even their units of measurement are different.

It is not generally understood that the recoil process begins immediately on firing, after the shot moves forward from the cartridge inside the chamber. This is because the high pressure gas of the burning propellant forces the projectile to accelerate along the barrel until it exits at the muzzle. Of course, at the same time, this gas pressure acts backwards against the gun to produce recoil (action and reaction).

Imagine if we suspend the gun by wires from the ceiling and fired it very carefully so as not to disturb it in anyway, then we would see the gun swing backwards a considerable way. An even better test would be to fire the unsupported gun in the frictionless vacuum and near zero gravity of outer space, so that we would see both gun and projectile move off in opposite directions at constant velocity for eternity - or until we halt them that is.

To stop the gun we must apply a braking force to the butt and we can choose to brake either quickly or slowly depending on what we need to do. The rate at which we stop the gun (deceleration) depends on the force applied to the butt - faster braking requires a higher force and is analogous to braking a car normally or very hard, as in the case of an emergency stop.

In reality, when we fire the gun, our shoulders (and other parts of the upper body) apply the braking force necessary to stop it in time for it to be moved forward into the ready position again. If we try to do this faster then we need to apply a larger braking force. The extreme case would be to place the butt against a solid brick wall when firing a shot. But, don't try this, as the resultant braking force would be so high that there is a real risk of breaking the stock.

Generally, when taking a shot, we don't make a conscious decision to use any particular braking force. In fact, it is largely determined for us by a set of parameters over which we only have a limited control. Therefore, the braking force which we apply to the gun through our shoulder, hands and arms is a reaction to gun recoil; so let's agree to call it a recoil reaction force or simply the reaction to signify the difference from recoil itself.
Even when the shot has left the barrel (and for some time afterwards) the high pressure gases escaping from the muzzle, together with the inertia in the rearward gun motion, continue to push the butt into the shoulder.

Reaction is dependent on a number of parameters such as the stature or build of the shooter, mass (weight) of gun, type of pad or clothing, ammunition and, of course, whether or not a recoil reducer system is used. A higher or lower reaction force occurs with respect to a variation in any one or combination of parameters such as these.

How are recoil and reaction measured?
Recoil is simply the maximum energy (foot-pounds [ft. lb.] in imperial units or Joules [J] in modern SI units) attained by the gun after firing a shot. Methods of measuring recoil and attempts to standardise it have been going on for over a century and authors such as W. W. Greener, Major Gerald Burrard and Major General Julan S. Hatcher have written much on the subject. Measurements like the pendulum gun, gun rest and spring dynamometer relate recoil to how much a gun recoils backwards and they are still in use today. More sophisticated tests, using dynamometers with electrical sensors, began in the 1920s through the quest for ever more accurate and lighter military rifles.

The human sensory system is not equipped to sense energy and so a recoil measurement is only an indication of what reaction a shooter can expect to feel when taking a shot with a gun. Our nervous system reacts instead to an applied force, which is expressed in units of pounds force [lbf] or Newtons [N] in SI metric. I'll give you values in both (remembering that 1 lbf = 4.448 N). A large recoil would only indicate that we should expect a large reaction at the shoulder.

Today, specially designed high-speed transducers are fitted onto the stock in order to measure and record the actual reaction force between shoulder and gun. Test results of this show that the reaction force varies considerably during a shot and is not just a simple push into the shoulder but a complex series of pulses occurring over a fraction of a second. Unfortunately, our nervous system is not sensitive enough to distinguish between these individual force pulses, which we perceive to be a single kick.

As soon as the gun is fired the reaction force increases rapidly against the shoulder to a peak value. This peak reaction force quickly falls off, but may be followed by several lesser peaks and troughs before returning to the pre-shot level again. The whole recoil event for a shotgun is completed in about 1/40 of a second. We can simplify things and talk about the reaction in terms of an equivalent mean or averaged force acting for the same amount of time as the real recoil. Typically, a mean reaction force delivered to the shoulder (per shot) from a 12 gauge shotgun using standard 28g load cartridge is about 350 N (approx. 80 lbf) but the peak force may be around 1200 N (approx. 270 lbs force) or higher; more than three times greater than the mean. If 1200 N force (equivalent to nearly five 56lb. sacks of potatoes) were to be applied slowly to your shoulder then I'm sure most of us would become uncomfortable very quickly. However, the peak acts for a considerably shorter time than the total recoil process itself; less than the blink of an eye in fact, and the body reacts in a different way to this kind of stimulus.

To consider only the mean force of the reaction is a gross over simplification. Only by accurately recording and analysing the rapid recoil pulse can we begin to understand the complex interaction between gun and shooter. The reaction is unique to the particular shooter and gun combination so that we can idealise it as a recoil signature, with no two shooters being alike.

Why do some cartridges kick much more than others?
Higher velocity cartridges of the same shot load will produce a higher reaction force at the shoulder. But, even when the velocity of two entirely different cartridges is similar, shooters may perceive a difference in kick. The explanation is usually found in the signature, which would have the same mean value but a completely different set of peak forces. Peaks in the signature contribute significantly to the difference in 'feel' between cartridges. Faster burning powders are one cause for this as they produce higher barrel pressures more rapidly than a slower type even though the two may have equal amounts of energy. A good comparison is the difference between the old slow burning black powder cartridges and the modern fast nitro based ones, where the boom of the former compared to the crack of the latter gives an audible indication of rate of burn.

How does gun weight affect recoil?
The so called conservation of momentum theory applies here, which states that the mass (weight) of the projectile times it's velocity must equal the mass of the gun times it's rearward velocity. So if we increase gun mass there must be a reduction in rearward velocity. Therefore, if we can reduce gun velocity then we reduce the reaction (braking force) necessary at the shoulder to slow it down and bring it under control. Near zero recoil could be achieved with a very heavy gun mass but clearly, for practical reasons there needs to be a trade off between good handling and recoil reduction.

In practice, the mass of a gun can be increased by including the mass of the shooter's arms (plus a fair portion of the upper body) through having a good grip with the hands and ensuring correct placement of the butt into the shoulder. A correctly fitting gun is important from this point of view and military style pistol grips can contribute significantly to reducing recoil, although, these may not be aesthetically appealing to the field shooter.

Do gas-operated semi-automatics reduce recoil?
It is difficult to generalise about this but, certainly, the rearward motion of the gas operated bolt and reduction in barrel pressures contribute to an apparent reduction in recoil in this type of semi-automatic. Measurements show that, when compared with fixed breech guns (with the same shooter), the gas-operated semi-automatic usually has a lower overall mean reaction.

However, closer analysis of the signature may show that the semi-automatic vibrates considerably more during firing. This is due, mainly, to the mechanism of the semi-automatic being made up of several large parts that are free to move about. When the gun is excited by the sudden impulse of the recoil, these components vibrate independently (rather like jumping up and down with a lot of loose change in your pocket).

These additional vibrations, can give the gun a harshness when shooting that may be more pronounced after a large number of shots have been fired quickly. The shooter might feel a slight tingling sensation in the hands or shoulder. In the case of a poorly manufactured gun, where the components have not been fitted together with great precision, significant vibrations will occur that are very apparent in the recoil signature.

What are the effects of recoil?
Unlike other joints in the human body, the shoulder is not a simple ball and socket but a complex mechanism, which is made up of several bones that move together as a whole in order to provide the necessary articulation. Several muscles groups are used to actuate the shoulder and two important ones are the pectoralis major and minor that cross over each other (major over minor) to form a convenient pad of tissue on which we place the butt of a gun.

As we mount the gun in readiness to fire, the shoulder holding the stock is articulated in such a way that the pectoralis muscles are in a relaxed state and, when the gun is fired, the recoil applies a localised impulse pressure that tries to stretch them. The human body is not passive and if a force stimulant is severe enough then self-preservation takes over and an involuntary reflex reaction occurs over which we have only partial control. The process of stimulation and response takes time and does not come into full operation until well after the shot has left the barrel. Therefore, during a shot, the human body can do little to defend itself and we are open to the full attack of the recoiling gun.

The long term accumulative effects of recoil on shooters are not well documented and so it would be foolish to suggest limitations on the amount of shots we should make per day, week or year. For sure there is a diversity of injuries from simple headaches, bruising and backache to broken shoulder bones but, as you will now understand, there is an equal diversity of shooter and gun combinations that may be to blame.

There is no simple answer to the question of accumulative effects of recoil and how best to assess it. It is likely that a significant contribution to any discomfort and injury is due to the peak force in the early stages of the reaction. This catches our nervous system off guard well before our reflex has time to operate and can set up serious shock waves in the arms, shoulder and back; high-speed cine recordings confirm this.

What steps can I take to minimise the effects of recoil?
Like all physical sports involving impact or impulse, there are methods to minimise the risk of injury. In the absence of defined criteria we must take a common sense approach and consider all aches and pains as being early warning signs of something we are doing wrong in shooting. Appropriate action must be taken sooner rather than later and, certainly, a proper gun mount and stance can make an immediate improvement in comort.

The peak force, mean force and duration of the reaction are important factors for comfort and safety. Methods such as using muzzle brakes or low velocity cartridges tackle the problem at source by reducing the recoil itself. If this is not practical then the use of either a proper recoil pad, mechanical absorber or a heavier gun will all have some effect to reduce recoil reaction forces.

Are recoil pads effective?
Recoil pads made of materials such as rubber or sorbothane can offer some help in reducing the effect of recoil on the shooter. Their main effect is to reduce the peak force of the recoil but they do little to reduce the overall mean. Sorbothane is a rubber-like material with very good energy absorbing qualities that make it useful as a shock absorber. A lot depends on the design of the pad, for example, rubber pads with a honeycomb type internal structure can compress more easily than a solid one.

However, if the pad is too soft then the gun will accelerate rearwards more easily into the shoulder while picking up extra momentum on the way and, therefore, a greater reaction force is required to stop it.

Will extending the forcing cones, or overboring barrels reduce recoil?
Barrel overboring or extending cones may improve patterning performance and this can easily be tested down at the pattern plate. It has also been suggested that these modifications reduce recoil and recoil reaction but, when pressed, manufacturers have amended this and say that they affect only the perceived recoil not the actual one. Confused? I will attempt to clarify.

Barrel overboring means to bore out the barrel from a standard 0.729" diameter to, say, 0.740" diameter. In fact, what we are doing is increasing the bore cross-sectional area and so, assuming that the gas pressure would remain unchanged and without doing any fancy measurements, simple theory would predict that overboring should actually result in a slight increase in the recoil and reaction force. And so it is; experiments with the same gun and shooter show a small increase in the recoil reaction (mean) of about 2% to 3%; similar to the increase in bore cross section.

The cone lengths on a 12 gauge shotgun can vary between 10mm to 50mm (3/8" to 2") or so, although, many manufacturers choose to chamber their guns with short cone lengths of 10mm to 16mm (3/8" to 5/8"). The purpose of the cone is to offer a smooth passage of the shot charge as it emerges from the cartridge and travels into the barrel. However, in doing this we have again increased the cross section of the bore albeit for a short distance; and so once again, experimental results show a small increase in the reaction at the shoulder when comparing with the same shooter and gun combination. An interesting feature of using extended cones is that, apart from this small increase in the reaction, there is little difference in using either an extended or short cone on a well-made gun. However, if, as I have found on some guns, the chamber and cone are not in perfect alignment with the barrel, then, effectively, the wad and shot charge have to turn a little to realign themselves as they emerge from the cartridge case and enter the barrel. This causes a delay in the passage of the shot and, as a result, there is some increase in barrel gas pressure. The result is a significantly higher initial peak in the recoil signature. Extending the cone length removes some of this problem and may improve shot patterns, but, for entirely the wrong reason.

Overboring and/or extending cones can contribute to a different feel or 'perceived recoil' from the gun but they do not reduce recoil. Overboring may also influence the perceived recoil more by altering the natural vibration properties of the gun. This is analogous to a bell, which is tuned by removing small amounts of metal from it to give a different note.