AR-15 Architecture: The Key To Function

BY Herschel Smith
1 month, 2 weeks ago

Shooting Sports USA.

Armalite’s original blueprints specified a 20-inch barrel, 12-inch (measured from the upper-receiver face) gas-port location and a ballpark 12,500 PSI gas-port pressure level. Those are the “rifle” specifications, which work just fine. Therefore, the problem isn’t in the design. Rather, it’s in the redesign. The shorter the front end is, the more redesign has its influence. Carbine-length barrels and especially pistol-length barrels create a condition where using the same ammo there is higher gas pressure at the gas port.

An AR-15 has a “direct impingement” system. Propellant gas is bled off through a gas port in the barrel, this gas goes through the gas manifold or “gas block,” through a gas tube, and into the carrier key atop the bolt carrier, and that’s the end of the line—the resultant force deposited into the key starts the bolt-carrier assembly in motion.

Think of this gas system as a pressure chamber with two valves—one at the gas port and the other at the muzzle, and the moving bullet opens each valve as it crosses each plane. Therefore, the spacing of the valves matters greatly.

The better way to think of this is each is an orifice (even though an adjustable gas block is an adjustable orifice), not a valve.

As the bullet enters the barrel bore and moves forward, the space (volume) behind the bullet is increasing, which lowers the pressure of the contained gas behind it. More available barrel-bore volume before the gas port translates to lower pressure at the port. So, post-gas-port barrel length influences how long the system is “sealed” under full pressure, and the shorter the length, the shorter time. Again, the bullet is serving as a plug that’s sealing all the pressure in the system—until it exits.

[ … ]

Approximate figures for M855 (genuine 5.56 NATO) chamber pressure is about 60,000 PSI; pistol-location gas port pressure, 50,000; carbine-location, 33,000; mid-length, 27,000; rifle-location, 19,000. Thus, 5.56 NATO is hot and getting hotter, and it has been for years.

Installing a heavier buffer and also a stouter buffer spring buys time. Both increase resistance to the bolt unlocking, thus delaying it from moving. The heavier buffer better resists movement and moves slower. The stouter spring increases in-battery load against the bolt carrier, increasing its resistance to initial movement, and the carrier also then moves slower coming back against the buffer.

“Just put an adjustable gas block on it” is also routine advice, and, yes, that helps, but ideally such devices should be used to tune function. If it’s needed just to make the gun run, then the chances are certain something else was missed, most likely in the architecture. That’s where we’ll find the cure.

[ … ]

That’s easy enough with a carbine-length front, 16-inch barrel. A mid-length gas port is located 2 inches farther ahead of carbine-standard. It effectively also shortens the post-port distance by the same amount meaning lower pressure getting in and a shorter time the system is under maximum pressure. Both are good things.

If you’ve got a hankering for an AR-platform pistol, choosing a 10.5- or 11.5-inch barrel makes it possible to get a carbine-length gas system affixed, and that is a 3-inch additional length over the common 4-inch pistol-port, and a significant reduction in post-port length. That really tames a little gun.

I think it’s becoming fairly routine advice to choose the right gas system for your gun as a first step to correct over- or under-gassed systems, rather than tinkering with the gas block.

This is what the author is calling the “architectural” solution to the problem.  This isn’t exactly the same thing as having an adjustable gas block on a new AR-10 6.5 Creedmoor with a fixed 20″ barrel for folks who want to shoot hotter loads or those who want to hand load specialized cartridges and need to make slight adjustments on the block to ensure reliable operation.


Comments

  1. On February 20, 2020 at 7:33 am, ragman said:

    I “tune” my ARs with a combination of full auto BCG(heavier), Sprinco buffer springs and Spikes buffers. When done properly the brass ejects reliably at 4 o’clock and locks the bolt back after the last round. The rifle shoots softer, cleaner and the components should last longer. I also install a Sprinco 5 coil extractor spring for enhanced reliability.

  2. On February 20, 2020 at 12:28 pm, TRX said:

    Armalite is still around, and they have some interesting technical papers on their web site. One of them is about barrel length, gas pressures, and reliability, and how when the Army shortened the barrels to make the M4 and then ran back to Armalite whining about reliability problems, their reply was “We told you not to do that.”

  3. On February 20, 2020 at 12:45 pm, Herschel Smith said:

    @TRX,

    So drop us a URL on said paper.

  4. On February 20, 2020 at 2:50 pm, Double Barrel In My Jacket Sleeve said:

    A gearmaster bud has a one of a kind AR with different operation from gas system and a custom barrel, it even has a spiffy Punisher logo.
    I said that is a piece of art but you could have bought a case of regular ARs and arm up a squad when Timey Spice arrives.
    He hasn’t called since after Christmas.
    I love the AR platform for low recoil, light weight, accuracy and ability to saturate a target but I prefer something more rugged for the Loogaboo.
    Just two cents but my favorite battle rifles are the M-14 and the SVT-40

  5. On February 20, 2020 at 3:54 pm, Georgiaboy61 said:

    @ TRX

    Today’s Armalite is owned by former Army Ordnance officer Mark Westrom. Neither he, nor his company, have any connection to the Armalite which in the 1950s was a division of Fairchild Aircraft and where Eugene Stoner, Jim Sullivan, et al. were employed at the time the original AR10 and AR15 were designed. Without question, today’s Armalite is a fine company with high-quality products, but they weren’t present at the creation. Just an FYI in case you didn’t know….

    @ Herschel

    Re: “I think it’s becoming fairly routine advice to choose the right gas system for your gun as a first step to correct over- or under-gassed systems, rather than tinkering with the gas block.

    This is what the author is calling the “architectural” solution to the problem. This isn’t exactly the same thing as having an adjustable gas block on a new AR-10 6.5 Creedmoor with a fixed 20″ barrel for folks who want to shoot hotter loads or those who want to hand load specialized cartridges and need to make slight adjustments on the block to ensure reliable operation.”

    Roger that -thanks for the info….

  6. On February 21, 2020 at 8:40 am, Name (required) said:

    Never liked the .223 or the AR platform, settled on the FAL when parts kits and mags were cheap. Still happy with the choice.
    I recently bought an AR10 lower, plan to put a 6.5 Creedmore upper on it. Figured after 50ish years, it’s time to give the platform a try. Perhaps it will be more than a toy.

  7. On February 21, 2020 at 11:19 pm, X said:

    Personally I have always preferred the A2 20″ rifle configuration. It’s kinda funny to hear these young kids call the A2 a “musket” (they should try humping an M1 Garand sometime).

    But I do admit that 16 inchers are handy lil’ buggers. But my 16 inchers are midlengths, not carbines, with M2 or M3 buffers depending on port size. Works real nice. I like the additional grip area of the longer forend.

  8. On February 22, 2020 at 12:38 pm, Ned said:

    The gas travels through the gas key – not to the gas key – to the gas piston (bolt) in the center of the bolt carrier, unlocking the bolt.

    The bullet travel past the gas port is dwell time.

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You are currently reading "AR-15 Architecture: The Key To Function", entry #23409 on The Captain's Journal.

This article is filed under the category(s) AR-15s,Firearms,Guns and was published February 19th, 2020 by Herschel Smith.

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