M16 and M4 weapons – the military versions of the AR15
The AR15 rifle platform has now been in military service for 50 years, and it’s come a long way since the US Air Force bought a few small batches to issue to its policemen in Vietnam. The basic M16 and M4 variants have been produced in the millions, mostly by Colt and FN, for service with the American armed forces and as military aid to US allies. Canada adopted improved variants made by Diemaco, and these have been an export success; as well as being the standard rifle of the Netherlands, Denmark and (formerly) Norway they have also been bought in some numbers by the British SAS.
There are many reasons for the popularity of this basic rifle design. It’s light, for a start; even the full-length M16 version weighs in at less than 7.5 pounds unloaded. Compared to the 7.62mm weapons it replaced in many armies, which generally weighed between 9 and 10 pounds, that’s not a lot. It’s also simple to maintain, reasonably accurate and can be fitted with a wide range of accessories. The short-barrelled M4 models aren’t effective much beyond 150 yards, but the M16 performs fairly well out to over 300. Once the rifle is loaded and cocked it benefits from good ergonomics; the change lever can be switched from safe to semi-automatic with the thumb of the trigger hand, so there’s no need to use the trigger finger as you have to with Britain’s clunky SA80 system.
The AR15 platform does have one drawback though; its reliability is average at best. The early M16s gained a terrible reputation in Vietnam, but this was later traced to lack of cleaning equipment, a change to a dirtier ammunition propellant, inadequate training in weapon maintenance and bad drills by poorly trained and unmotivated conscript troops. The rifle itself was acceptably reliable, and performed much better once the Vietnam teething troubles were overcome. It’s never been any more than acceptable, though, and that’s due to an integral feature of the design.
Almost uniquely among gas-operated weapons the AR15 design uses what’s known as a “direct impingement” gas system. When any gas-operated rifle is fired high-pressure gas is tapped from a small hole in the barrel and fed backwards down a tube. Generally it strikes a piston and drives it back forcefully; the piston unlocks the rifle’s bolt and pushes it back, ejecting the spent round and chambering a new one. There are many variants – short and long stroke pistons, for example, or the Kalashnikov design where the piston forms a long forward extension of the bolt – but the basic principle is always the same.
Not in the AR15, though. Gas is still tapped from the barrel and fed down a tube, but there’s no piston. Instead the gas travels all the way back into the action of the rifle, where it acts directly on the bolt to drive it back. This system is easy and cheap to manufacture, and it slightly reduces the mass that’s moving around while each shot is fired; in theory that should increase accuracy. It also reduces the number of parts in the weapon and cuts down on cleaning. Speak to anyone who’s ever used an L1A1 SLR or FN-FAL and you’ll hear just how dirty gas parts can get.
Of course there’s a reason why gas parts get dirty. That propellant gas is hot and it’s full of carbon compounds and unburned powder. It’s filthy stuff and usually it transfers that filth to the gas parts, turning them into blackened horrors that take ages to clean. In the AR15 platform, though, all that hot dirty gas is blasted right into the weapon’s action – and that’s a problem.
If a collection of oiled metal parts is regularly blasted with filthy, searing hot gas the lubrication will eventually be burned away. If the metal is then being simultaneously heated up and coated with abrasive grime the weapon is going to start to suffer stoppages. That’s what happens in an AR15. The basic design is good enough that it usually keeps working, but the direct impingement gas system means that reliability is fundamentally compromised.
Despite this issue the AR15 platform remains popular. It’s largely unaffected by adverse climates and millions of troops in over 80 countries are familiar with how it operates. That creates some big incentives to improve its reliability. In a future article I’ll look at how Heckler & Koch have done exactly that with their AR15-derived HK416 carbine and its big brother, the handy and powerful HK417.