An Cosantóir

www.military.ie THE DEFENCE FORCES MAGAZINE | 31 BY CAPT ORDNANCE SCHOOL, DFTC R ecoil in a rifle is a force that pushes the weapon back to- wards the firer. This force is created when a round is fired. The Law of Conservation of Momentum is a familiar concept to us where every action must have an equal and opposite reac- tion. The action of a rifle round being fired typically involves a firing pin striking a cartridge primer, which initiates the deflagra- tion (extremely fast combustion) of the propellant grains in the cartridge. As the propellant burns, the rapidly expanding hot gas generates a massive amount of pressure. For SS109 5.56x45mm ball, this pressure is approximately 430 Mega Pascals (MPa). To put this in perspective, that is the pressure you would feel if you were under 43km of seawater! This high-pressure gas can only be released in one direction - down the barrel. The expanding hot gas pushes the bullet from the cartridge case through the barrel and into the air. The same force that drives the bullet from the barrel produces an equal-but-opposite force, which drives the butt-stock into the firer's shoulder. Using Newton's laws of motion, it is possible to determine exactly how much recoil is created when a round is fired in a rifle. When conducting these calculations, it is necessary to consider recoil in terms of energy. Thus, it is called the free recoil energy, which is a measure of kinetic energy. Kinetic energy is the energy of a moving thing and is measured in joules (J). The free recoil energy of a Steyr AUG A1 Assault Rifle firing SS109 5.56x45mm ball ammu- nition is 4.62 J. In comparison, the free recoil energy of a Steyr AUG Mod 14 Assault Rifle firing the same ammunition nature is 3.92 J. This makes sense as the Mod 14 is a heavier weapon, it takes more energy to move it and so the firer feels less recoil. In fact, the firer of an AUG Mod 14 will experience 15% less recoil than the firer of an AUG A1. Coupled with the improved optics of the Mod 14, this has the potential to increase a firer's accuracy and consistency. Understanding how much recoil is created by each shot allows weapon engineers to design rifles, which reduce the recoil felt by the firer. This can be done in a number of ways. The basis of each method, however, is to get the recoil force from the round to do some other kind of work than simply slapping the weapon into the firer, as is the case with a bolt action rifle. The energy from the recoil force can be converted into doing more useful work such as extracting a spent casing, compressing a return spring (allowing for automatic cycling of a weapon), and recocking a hammer. Alternatively, the propellant gases can be used to do work to counter the recoil force through the use of a muzzle brake or suppressor. These actions will reduce the recoil felt by the firer. In rifles where the butt-stock is in-line with the barrel, the recoil force is sent directly back into the firer. The Steyr AUG has an in-line butt-stock. Where the butt-stock is not in-line, as in the FN FAL or AK47, then the recoil force creates a turning moment when a round is fired. This occurs because the anchor point of the butt-stock on a firer's shoulder is below the barrel. Firing causes the weapon to rise. Weapon engineers attempt to counter such a turning moment by making in-line stocks or by deflecting propellant gases upwards at the muzzle to push the muzzle downward. The addition of an internal mechanism to absorb some of the recoil means that the weapon will not just have a single impulse of recoil. First, the firer will experience the recoil from the round be- ing fired, then they will feel the recoil of the weapon's mechanism striking the weapon's casing or buffer spring. These impulses typi- cally occur within 0.03s and so are almost impossible to sense. This second impulse will be far smaller in intensity than the first, unless a weapon is unbalanced. In an unbalanced weapon, too much force is being recovered by the gas system and transmitted to the inter- nal mechanism. This has the effect of slamming the mechanism into the casing or buffer spring with too much force and reduces the weapon's accuracy.

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