The two primary calibers for small arms that are NATO standards are the 5.56 x 45mm and the 7.62 x 51mm and they have their civilian equivalents. The 5.56 x 45mm is the .223 Remington while the civilian form of the 7.62 x 51mm is the .308 Winchester, and though they share cartridge dimensions they are not necessarily the same.

Measuring Free Bore & Chamber Design

First there is the concept of the chamber design and part of that specification is free bore. Free bore is the distance between the ogive of the bullet and where the rifling starts. In most civilian commercial designs, the bullet is in very close proximity to the rifling with little free bore. Not so with the NATO military firearms. Both NATO rounds are designed with some free bore. Military weapons must accommodate different bullets, generally tracers are longer than the standard ball ammo in the smaller caliber. A variety of special purpose ammo from ball, armor piercing to blanks are fired from these weapons. The bullet jumps a gap on firing of the cartridge before engaging the lands of the rifling. What that means is that there is a slight pressure drop until the propellant gases push the bullet into and engage the rifling where pressure goes up. This allows the bullet to build a little inertia before engaging the rifling. In civilian chamberings the bullet is in contact with rifling and the peak pressure arises early on as the gases must overcome the cutting of the lands into the bullet and overcoming the resultant friction. Another aspect is that the sports SAAMI chambers are tighter, enhancing accuracy, with less cartridge case expansion. Military chambers are ‘looser’ to promote more reliable operation in military select fire weapons. The effect is that a load that performs safely in a chamber with free bore, will produce higher chamber pressures in a tight chamber with little or no free bore and can be dangerous to the gun and shooter.

Military Grade VS Civilian Ammo Pressure

Point two is that most military loads operate at higher pressure than their civilian counter parts. Therefore, you can shoot your sporting loads in a military rifle, but not the NATO rounds in a civilian gun, unless you have a gunsmith add some free bore. Military ammo uses military specs using a different test procedure than SAAMI, with NATO pressure rating appearing more conservative than SAAMI. NATO ammo is fired from looser chambers to ensure extraction during the firing cycle of semi and full automatic weapons. Not a concern for the sportsman, with the camming action of the bolt action rifle. The point is to use CAUTION when firing NATO spec ammo in civilian weapons. High pressure and possible mechanical failure of the action is possible due to high pressures of the military ammo. But the civilian spec ammo can be shot in military actions, at a slight velocity loss due to the relatively long free bore of the military weapons. Numerous manufacturers of the AR-15 offer their guns in civilian chambering using SAAMI not NATO specs. Be careful. Remember that the military ammo produces similar pressures to the civilian loads, but this is despite the free bore. They are hotter in a civilian chamber spec.

Custom Shooting Ranges for Firearms Practice & Training

So, don’t mix your military ammo with the sporting numbers, it can be disastrous. For the hand loader, remember military brass tends to be thicker in walls and they hold less. A rule of thumb is to reduce loads 10% to compensate. Know your weapon, its characteristics and specifications. Shooting Range Industries designs and builds custom modularshooting ranges to best suit your needs. Contact us to learn more today.