Mid-Length Best Length? NSWC-Crane Tests Say Yes

A Geissele Automatics commercial USASOC mid-length upper receiver. (Photo: Brownells)

Researchers at the Naval Surface Warfare Center Crane Division — NSWC-Crane — are putting a long-running debate to rest. Is the standard carbine-length gas system the best? Or is there an advantage to mid-length?

Specifically, the testers wanted to see if the mid-length gas system showed significant advantages with shorter 14.5-inch barrels. But these findings apply as much as, or more for, 16-inch barrels which are the most common on AR-15 rifles today.

The carbine-length gas system was designed to cycle a wide range of barrel lengths and profiles. Even though it’s a standard many companies and shooters alike disagree, leaning toward mid-length.

In recent years boutique builders and prominent manufacturers started using the mid-length gas system, not just on 16-inch carbines, but also 14.5- and 14.7-inch setups.

The supposed benefits of the mid-length setup include lower running temps, shorter dwell times and less gas pressure on the system. The potential drawback is lower reliability — after all, it’s not the norm.

But because the carbine-length gas system is capable of cycling guns with barrels starting around 10 inches, it shows signs of over-gassing guns with longer barrels. Many users turn to extra-strong buffer springs, heavy buffers and enhanced bolt carriers to improve performance with their carbine-length gas systems.

NSWC-Crane started a series of tests to determine what the actual benefits and drawbacks of using mid-length gas systems with 14.5-inch barrels. And while they haven’t published all the results, initial tests confirm what a lot of shooters have been saying.

Mid-length looks a lot better with 14.5-inch barrels. Again, these results also carry over to 14.7- and 16-inch barrel rifles.

NSWC-Crane tested for overall reliability, high- and low-temperature reliability, accuracy, projectile velocity and cyclic rate. They also performed tests suppressed as well as unsuppressed.

In most cases, the mid-length gas system outperformed the carbine-length system. The mid-length system matched or bested the carbine-length system in terms of muzzle and terminal velocities, cycled more slowly — slower is better in this case — and the mid-length system had far fewer malfunctions.

All of the testing used the new M855A1 Enhanced Performance Round. This is very high-pressure ammunition and is considered a “barrel burner” since it puts a lot of wear on rifles.

See Also: NSWC-Crane Shows M-Lok Takes the Advantage

While it was nice to find that the mid-length system malfunctioned half as much as the carbine-length system, NSWC-Crane’s real eye-opening finding was that the mid-length gas system showed a far better service life than the carbine.

Crane researchers started to see a degradation in accuracy and performance at around 6,000 rounds with their carbine-length setup. But they saw no drop in accuracy with their mid-length setup even after over 12,000 rounds.

So is there any benefit to carbine-length gas systems on 14.5-inch barrels and longer? Looking at these results, the answer is not generally.

Obviously the carbine-length gas system is still useful for shorter barrels, and it’s useful for other cartridges and loads. But for full-house 5.56 carbines, mid-length appears the way to go.

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About the author: Max Slowik is a writer with over a dozen years of experience and is a lifelong shooter. He has unwavering support for the Second Amendment and the human right to self-defense. His ambition is to follow Thomas Paine, as a journalist by profession and a propagandist by inclination.

{ 11 comments… add one }
  • Mike Watkins May 28, 2018, 12:35 pm

    I’ll leave all you AR owners to debate over whether a mid or carbine length gas system is more reliable.

    Excuse me, got to go dig my AK out of the mud and shoot some bad guys.

    Seriously, I’m an AR fan, got every length gas system made, got quite a few dif calibers. But AR’s, even the best of them, jam. I don’t really know all the reasons, but it’s probably from the same characteristics that make AR’s more accurate: tighter tolerances, etc.

    You say yours doesn’t EVER have an ftf, fte, smokestack, etc? Fire some more rounds. Fire under adverse conditions where you don’t have the opportunity to clean. You WILL have stoppages.

    I went to war with an M16 when they were still ironing out the biggest issues. Yes, they’re better now, but still not 100%.

  • jgwills May 25, 2018, 2:36 pm

    I have a 14.7 carbine and 14.7 midlength upper from Palmetto, both using FN made barrels. Over all I prefer the middy for a variety of reasons. It runs softer, more handguard real estate, and I can go longer with my gas rings in the carrier. I also should mention I have a Franklin binary so they’ve both been run very hard. I need to note that I run with Franklin’s included buffer spring they recommend in the lower for more reliability with the binary trigger system. With the middy upper I never have the hammer follow issue. The carbine one will do it every now and then if you try to run the trigger too fast. With the mid length system running the carrier a bit slower I feel that its the way to go if your running it hard with a binary system. In my experience the mid length systems the way to go.


  • Jake May 25, 2018, 12:46 pm

    The mid length is far better for shorter barrels. The carbine length recoils at 240 fps. The mid length at about 200 fps. The rifle length at about 160 fps. The longer the gas tube, the lower the heat entering the bolt and the slower the bolt recoils lessening the pounding it takes over time.
    Most of us don’t shoot enough for it to matter. If you are replacing barrels or buying a new AR it would absolutely make sense to get a mid length simply because it is better.

  • kyle May 25, 2018, 9:56 am

    The supposed benefit of mid length over carbine length is longer dwell time? Sounds like the author, and who ever was advising him, has no idea what dwell time is.

    • AKB May 25, 2018, 11:45 am

      Ok, so explain to us all what dwell time is, if the author is so incorrect…

      • kyle May 25, 2018, 2:48 pm

        Dwell time is the time period that the gas system is being pressurized, which is the time during which the bullet is between the gas port and the muzzle. The closer the gas port is to the muzzle, the shorter the dwell time.

  • Jay May 25, 2018, 8:41 am

    No published results means an ad to sell mid length carbine parts, uppers and rifles! Why even write the article without the end results? hummmm??

    • Nick M May 25, 2018, 11:08 am

      The results are available online. You just have to be careful with some forums with an agenda as you are mentioning. Or just bad info like AR15.com.

  • Bob May 25, 2018, 8:26 am

    The results might be true but I would have to assume it was done with Full Auto Weapons. The average civilian Semi-Auto AR-15 is never going to take the abuse, if you will, of the rate of Fire of a fully automatic weapon and never hitting the same temperatures which will cause damage.

  • Cary Kieffer May 25, 2018, 7:50 am

    Interesting. I put an 11.5 Faxon barrel on a build. It also has a mid length gas, so apparently you can go even shorter to at least 11.5. The barrel is awesome. Runs like a swiss watch and if I do my part keeps 100% of the shots on a 11 inch steel plate at 400 yards with a 1-8x Strike Eagle mounted up top. I have quite a few of both length and mid always seemed like they were easier to rapid fire, a little less muzzle movement.

  • ToddB May 25, 2018, 7:26 am

    I made sure to dig up a mid length barrel when I did a build. Being a lefty, a slick side rifle with a carbine system, eating brass was an issue. Slick side mid length, not a problem. No crazy buffers or springs required.

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