M4 Mid-Length Gas System Better And More Reliable Than Carbine-Length Gas System
BY Herschel Smith
6 years, 5 months ago
Using a mid-length gas system on an M4A1 carbine extends the life of the weapon system and increases the weapon’s performance over a carbine-length gas system, according to a detailed study by Naval Surface Warfare Center — Crane, obtained by Military Times through a Freedom of Information Act request.
The Navy’s Crane center is responsible for testing, evaluating, procuring and managing the life-cycle of U.S. special operations forces’ weapon systems. So, naturally, they tested the mid-length gas system on M4A1 carbines at the behest of Army Special Operations Command.
This study may not come as a complete shock to civilian shooting aficionados and U.S. special operations forces who customize their M4 variants, but it does offer data to back up what those communities have believed for some time.
For the uninitiated, the crux of the issue comes down to when the M4 carbine first replaced the M16 rifle.
In developing the M4, the M16’s gas system was redesigned, according to Crane. The M16 uses a 20-inch barrel and gas system, but the M4 designs were crunched down to fit a 14.5-inch barrel.
Because of the shorter barrel, the gas port was moved down and the dwell distance — the delay between where the bullet passes the gas tube hole to the point where the bullet exits the barrel — decreased.
That decrease in distance from bolt face to gas port on the M4 resulted in an increased port pressure compared to the M16 of the past.
The M4’s port pressure measured at 17,000 psi, while the M16’s was at 10,000 psi.
Many civilian clones of the M4 utilize longer barrels, but also place mid-length gas systems on their custom-built designs. This customization increases the distance from bolt face to gas port than what would be normal on a standard issue M4.
Crane — located in rural Indiana — switched the carbine length gas system on the M4’s 14.5-inch barrel and upper receiver group with the mid-length gas system. Then the study cohort shot 12,600 rounds of M855A1 5.56mm through both designs for comparison testing.
The mid-length gas systems experienced a total of 30 malfunctions, while the carbine-length gas systems experienced more than double that at 65 malfunctions. Additionally, the carbine-length gas system suffered 13 unserviceable parts, while the mid-length gas system only suffered 9 unserviceable parts.
[Download the full Mid-Length vs. Carbine-Length Gas System report]
The study also found that the mid-length gas system experienced a decrease in bolt speed and a decreased cyclic rate of automatic fire.
The money quote is this: ” … but the M4 designs were crunched down to fit a 14.5-inch barrel.” And yet it didn’t have to be that way. My guns are all mid-length gas systems, and yours probably are as well.
As I’ve said, when you modify Stoner’s design you’d better be careful. He was a good engineer. Before modifying his design you need to be just as good an engineer as he was.
And it’s stated in the article and almost goes without saying that none of this is a shock to most of the gun community today. Once again the civilian gun community leads the way and shows the military what to do.
On June 4, 2018 at 2:56 am, ROFuher said:
Is “scrunched down” suggesting any thought was actually given to M4 gas system?
I would happily be corrected, but always understood the short gas tube was simply recycled from the Vietnam era Colt CAR. The tube length dictated the position of the gas port and sight tower. Since those had a bayonet lug, and it was an infantry rifle, the bayonet in inventory then determined the 14.5″ barrel.
Mid length gas tubes emerged during the ’94ban because the port pressures of a short tube and 16″ barrels are higher still, and without mounting a bayonet, the gas port could be positioned more sensibly.
Less gas pressure also has a softer recoil and assists with keeping the sights on target.,