Photo: Nick Jacobellis
Automaker Henry Ford once famously said: "The customer can have a Model T in any color he wants-so long as it is black." For decades that was also kind of the approach the gun manufacturers had to M16/AR-type rifles: The customer can have any operating system he wants as long as it's the direct impingement design of gunsmith and inventor Eugene Stoner.
But in recent years more and more manufacturers have developed new takes on AR-style rifles, resulting in four major operating systems for the common patrol rifles used by American law enforcement.
The result for some officers has been a cloud of confusion as to which AR operating system best fits their needs. So let's take a look at the four operating systems and their pros and cons.
Most M16s and the M4 carbines used by the U.S. military operate with a direct impingement gas system. In a direct impingement-powered M4, the hot gas from expended ammunition is deposited directly back into the chamber area of the receiver and is used to drive the bolt to the rear.
Unfortunately, in a direct impingement rifle or carbine, burned powder residue is also deposited into the upper and lower receiver area. This means that when a direct impingement M4 carbine is fired on a regular basis, the internal parts will heat up after being coated with hot gas and will also collect particles of burned gunpowder.
As long as a direct impingement rifle is properly maintained it will continue to function reliably. Which means, you have to keep it clean.
However, this statement is by no means meant to give you the impression that direct impingement M4s are fragile firearms that are prone to malfunction after a certain amount of use. Retired NYPD sergeant Pat Rogers of tactical training company E.A.G. Tactical has discovered that a well-made direct impingement M4 that technically needs a cleaning is capable of continuing to reliably operate when it is properly lubricated.
Mid-Length Direct Impingement
In the mid-length direct impingement operating system the gas system is a bit longer than the carbine length direct impingement gas system that is used for example in a Colt 6920 or a Colt 6940 M4.
The difference in length changes the dwell time, which results in a slightly softer recoil impulse. The end result is that a mid-length operating system is less punishing to the shooter's shoulder. An operating system that produces a noticeably softer recoil impulse also enables a properly trained and skilled operator to fire faster follow-up shots with more precision.
Note: A mid-length direct impingement operating system still needs to be properly maintained just like a direct impingement-powered M4.
When a piston-powered rifle or carbine is fired, the hot gas from the expended ammunition is used to power a steel rod or piston that pushes the bolt to the rear. Piston-powered rifles run cleaner and cooler than direct impingement rifles because the hot gas and particles of burned gunpowder from discharged ammunition are contained within the piston system. This prevents the bolt and the other internal parts in the receiver from heating up and getting dirty when the rifle is fired. In some piston rifles you can adjust the flow of gas from the expended ammunition.
Mid-Length Piston Power
The mid-length piston-powered operating system used in the LWRC M6A2 SPR or Special Purpose Rifle (M4) is exactly what the name implies. It's a combination of a piston system and a mid-length system.
According to an LWRC technician, "With the gas port being farther from the chamber and closer to the muzzle, the dwell time is shorter and thus the pressures in the system are reduced, reducing the perceived felt recoil by the shooter." Because the longer mid-length piston system shortens the dwell time, which in turn results in a softer recoil impulse, the operator of a mid-length piston rifle is able to make faster follow-up shots.
Shooting the Options
As someone who has field tested numerous select-fire and semi-automatic M16 and M4 variants, I recently began to wonder if there is any advantage to adopting a piston-powered M4, a mid-length direct impingement M4, or a mid-length piston-powered M4 over a traditional direct impingement design.
To conduct the test, I assembled a group of shooters and five semi-auto M4 variants. Our test weapons included a carbine length direct impingement powered Colt 6940, a carbine length direct impingement Colt 6920, a mid-length direct impingement Bravo Company EAG Tactical Model M4, a mid-length piston-powered LWRC M6A2 Special Purpose Rifle (M4), and a piston powered SIG 516 (M4). All rifles were tested using Winchester 55-grain 5.56 FMJ, Federal 55-grain 5.56 FMJ, and Israeli 62-grain military surplus ammunition.
Once the various M4s were sighted in using iron sights, Aimpoint red dot optics, and a Trijicon TAO1NSN ACOG magnified optic, we began the test. The evaluation was conducted from the standing unsupported position at a range of approximately 30 yards.
The first test compared the mid-length direct impingement Bravo Company EAG Tactical Model M4 to the mid-length gas piston-powered LWRC M6A2 SPR M4 to see if the mid-length operating system displayed a noticeably softer recoil impulse. Each member of the testing team fired numerous rounds from each M4.
Three out of four shooters immediately recognized that the mid-length direct impingement Bravo Company EAG Tactical Model M4 has a softer and shorter recoil impulse. All four shooters also noticed that the mid-length piston-powered LWRC International M6A2 SPR also has a soft shooting and short recoil impulse.