The link has a few stock photos plus a chart of cyclic rate for mid-length vs carbine.
This detailed report shows which M4 rifle design works the best
By: Kyle Rempfer
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.
The study also found that the mid-length gas system experienced a decrease in bolt speed and a decreased cyclic rate of automatic fire.
Crane found no statistically significant differences between the two gas system types in terms of weapon precision, barrel erosion, terminal velocity or muzzle velocity. Nor did Crane find any difference in malfunctions or rate of fire between the two gas systems when testing at high and low temperatures, which were 160 and -60 degrees Fahrenheit, respectively.
The Crane study concluded that the mid-length gas system was “recommended for use,” but also decided to continue testing. Crane plans to test the two gas systems up to more than 30,000 rounds.
Interesting, thanks for posting.
I wonder how many firearms were involved in the testing.
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The carbine gas "aficionados" are no doubt blowing a damn gasket right now on the "other" forums. They flat out deny that the mid-length is better. Most of us with a brain can see the benefits. Lower port pressure, less wear and tear, proper dwell time, easier extraction. Twice the reliability was a nice benefit as well.
Hopefully the military gets their head out of their ass pronto and some M4A? rifles hit the street. With new mid length barrels.
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Nice post and good to know!
It seems that they are comparing 2 rifles head-to-head. I really hope that they'll run something like 20 rifles head-to-head in order to make this more robust. Otherwise, this is pretty much an anecdote at this point. I.e., any of us with 2 rifles and 25,000 rounds of ammo to burn could do this test and share the findings.
I'd think that the military would jump at the opportunity to really test out wear and tear / longevity between the two gas lengths considering the stakes at hand, such as reliability, logistics of service times/frequency, parts usage, and ultimately, maybe even soldier survivability if rifles are being pushed very hard without optimal servicing.
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I know this is a recent article but this is really old news. ARFCOM has more than it's share of geardos, gidiots, and tacticool wannabes who will never accept anything that's not in the TDP, but the superiority of the mid length gas system is pretty proven science.
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It sounds like it's 3 of each. Here is another article: http://soldiersystems.net/2018...creased-performance/
Many of us have known about the benefits of mid-length (over carbine gas) for over 20 years. It's a shame how slow our military can be to catch on.
Yes, it's a benefit (and I prefer it), but still not substantial enough to get rid of a carbine you already have IMO.
Buying or building new? Yeah, go mid-length, there really isn't a downside.
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^ I agree. If you have a 6920, no worries, you’re good to go. Have a handful of carbine gas? Great! Want to pick up one or two middies? Go for it.
Either is fine, and the midlength may offer a few benefits that will slight increase bolt life plus offer a different (softer) recoil impulse.
I only have middies now, but I’d own a carbine gas if the price was right.
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Carbine gas is meant for 14.5" barrel. On a 16" it screws up dwell time as I understand it. Hence the reason for midlength. Thought it was that simple.
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That arrangement was settled upon to keep capability to fit a standard bayonet on an M4. Mid-length works well on a 14.5 as well.
I have owned both and I can feel the difference, but I wouldn't worry about it too much if you own carbine length.
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