An Overview of M4 Gas Operating System Not a DI

An Overview about M4’s Gas Operating System

            A soldier firing the M4 Carbine

The thing that move me to write this blog is most of the peoples are confused the Direct impingement system with the stoner M4 but actually the stoner M4 system is not a direct impingement. A gas operated system is a type of firearms action mechanism that uses the gases produced by firing a round to cycle the firearms action that is eject the spent cartridge, chamber a new round, and reset the firing mechanism.

Gas Generation system: When a round is fired, the expanding gases from the ignited gun powder push the bullet down the barrel.

Gas Port: A small hole, called a gas port, is located near the muzzle of the barrel.

Gas Diverted: Some of the gas from the firing process is diverted through this gas port and into a gas tube or pistons system.

Action and Cycling: The gas then moves either directly to the bolt carrier group (in a Direct impingement system). This gas pressure forces the bolt carrier to move rearward and unlock the bolt for ejecting the spent case and chambering the next round.

There are two primary types of gas operated systems:

• Direct impingement: Gas is directed into the bolt carrier group, directly cycling the action.

• Piston driven: Gas is directed into a piston that pushes the bolt carrier group to cycle the action.

But the thing is most people believe the M4 is a Direct impingement system but it is not, M4 is a gas operated cylinder driven system.

Let dive into this, I take two rifles for the example that is MAS 49 and the M4.

MAS 49 rifle, a french semo automatic weapon introduced in 1949, 

uses a direct impingement gas operated system paired with tilting bolt mechanism. When the rifle is fired, the gunpowder in the cartridge ignites, and the expanding gases from the burning powder push the bullet down the barrel, Near the muzzle of the barrel, there is a gas port that diverts a portion of these expanding gases into a gas tube that runs along the top of the rifle. This gas is then directed toward the rifles action. The gas enters the direct impingement system and is directed straight into the bolt carrier group. This is where the MAS 49 different from a piston driven system. The gas directly impacts the bolt carrier and forces it to move rearward. The spent casing is ejected form the chamber, the firing mechanism reset, The bolt is pulled rearward and then pushed forward to chamber a new round. Here the tilting bolt mechanism similar to SLR, the bolt tilt downward for locking and tilt upward for unlocking.

The below image shows the true DI of the MAS 49

The first figure shows the locked bolt before firing, second figure shows the unlock of the bolt due to the impingement of the gas. There is no expansion chamber, the gas tube mate with the hole in the bolt carrier. The gas is directly impinge into the bolt carrier hole.

The above figure shows the impingement hole in the bolt carrier.

The above figure shows the gas tube is aligned with the impingement hole in the bolt carrier.

When it comes to M4,

The figure shows the Bolt carrier group of M4

The gas piston is in the bolt and cylinder is in the bolt carrier, both are assembled together to make a bolt carrier group, in the time of assembling the bolt and carrier automatically it will make a gas expansion cylinder and piston setup inside the bolt carrier group. After firing, the trapped gases are guided through a long gas tube into the gas key. The gases then enter an expansion chamber inside the bolt carrier.

The cut view of the M4 bolt and carrier and gas expansion chamber in locked position.

At the rear of the bolt, there is a piston-like section with two gas rings. These gas rings seal the outer diameter of the bolt and the inner diameter of the carrier to prevent gas leaks during cycling and inside the bolt carrier there is a expansion chamber or cylinder. In the right side of the cylinder there is a two vent holes.

After firing, gas flows into the expansion chamber through the gas key and gas tube. Since the gas naturally tends to flow along the path of least resistance, it expands rearward because there is less resistance in that direction. This is due to the fact that the bolt is locked in place with the barrel extension and cannot move forward. So, the gas expands inside the bolt carrier, pushing the carrier rearward, while the bolt and the piston section is stays fixed. As the bolt carrier moves rearward due to the expanding gas, the rifle starts cycling. Before cycling, the gas vent holes in the bolt carrier are forward to the gas rings in the piston, and the gas rings in the piston seal the expansion chamber. After cycling, the bolt carrier and the gas vent holes move backward, and the expanding gas pushes against the fixed piston and move the carrier back (While moving rearward the piston and the bolt is fixed the carrier only moves), the rearward movement of the carrier will break the seal and venting the high-pressure gas out through the ejection port.

The below image shows the Gas operated movable expansion chamber fixed piston system in the M4 Carbine.

In simple terms, the piston and gas cylinder are built into the bolt and bolt carrier, unlike other rifles where they are placed near the handguard.

I hope the readers will enjoy this, let me know your thoughts in coments.