Skip to comments.AR-15 Gas Systems: The Complete Guide
Posted on 09/03/2019 11:29:18 AM PDT by Black_Rifle_Gunsmith
A regular AR-15 fires, ejects, and chambers rounds using either a direct-impingement or piston-operated gas recoil system. Thats a mouthful, lets just call it a gas system either way. Just like the AR-15 comes in a plethora of calibers and configuration, so too does its gas system. There is no real, universal logic for picking the right gas setup for your black rifle, SBR, or pistol.
Continuing with the general theme, this week's guide provides visual illustrations and an explanation of the AR-15 gas system. It includes comparisons between DI and piston systems, gas system lengths, types of gas blocks, and other technicals. If you're a black rifle owner and you deal with excessive recoil, failures to feed, or light primer strikes, this guide may be helpful. This is article is part of the Complete Parts Guide on Building an AR-15 with a stripped lower receiver or 80 lower receiver (unregistered receiver blank). To those of you bookmarking, hopefully this guide is also useful. If you have questions or want to discuss, feel free to comment. I'll do my best to answer or find answers.
Where does the gas system length measurement start?
Good article. I’ve saved it for reference. I like the animations.
If I understand what you’re asking, the length would be the same as that of the gas tube,from the BCA to the gas port.
That could be. However the diagrams appear to show measuring from the chamber end of the barrel extension.
I just buy the whole upper or the upper kit for the barrel length I want. No worries that way.
Del - Thanks for your help.
Everything I have works with a piston. No DI.
Just out of curiosity, is there a failure rate of the piston version in an AR? I have thought that the more moving parts, the more problems.
There's a chart in this article:
It can be a problem after many (thousands) of rounds. Pipe cleaner is about the best you can do for field cleaning. You could drop the entire upper into a solvent bath if you are back home from the range. I have put several thousand rounds through several gas system AR’s and have yet to have to take one apart to clean the gas system or even have to solvent bath one. But it could come to that if it gets dirty enough.
I think that there are more issues with conversion kits to change your DI rifle to piston than there is for a dedicated piston AR design such as a Ruger SR556 or HK416.
The main issue with conversions is that the AR "direct impingement" system is really a piston that is inline with the bolt face, so that the piston forces are straight back down the axis of the bolt carrier.
For a gas piston conversion, the DI bolt carrier's gas key is replaced with a solid surface for the op rod to smack, and that off-axis force causes the bolt carrier to tilt back, often putting wear on the buffer tube area.
Dedicated gas piston designs use a bolt carrier and upper designed to compensate for that offset whack by the op rod.
Gas piston bolt carrier:
Thanks for this post!
Saw super long pipe cleaners for sale somewhere intended for the gas tube.
Always thought of it as self cleaning anyway. Military seems to think that way.
You got it.
I've never had a problem.
Bolt and carrier stay cooler and cleaner.
Ive built several AR rifles, all are DI style. My problem with the piston style is one item. The piston systems arent mil-specd. I might consider building piston style but for that. I like putting the things together in various configurations. One of my favorites is a 6.5 Grendel. A friend I no longer have due to untimely death always said the ARs are like a redneck Barbie doll, you can dress her up anyway you like. Before he passed he built his wife one with a pink lower. Thanks for the info.
“What keeps the gas tube clean?
The cleaning instructions does not seem to mention it.
Or is it not a problem?” [Bitwielder1, post 10]
The system was originally designed to function with a specific load of a specific propellant, propelling a specific bullet to specific velocity. It’s pretty much the same with any self-loading firearm. And it’s the way the military has always required small arms of this type to work.
In practice, this means the gun will operate reliably when the ammunition uses only a relatively narrow range of propellant types, and bullet weights/types. Generally fine for military purposes, but when someone (often civilian users) changes propellant types or loadings, or bullets, they run into trouble quickly.
Early military users of the M16 ran into problems when the contractor loading ammunition substituted ball powder for the chopped-tube powder that rifle was originally designed to work with. Ball powder is produced in ways entirely different from the chopped-tube sort, with different additives. Function was greatly affected, fouling went way up, and gas tubes were clogged by residue from a drying and pH-balancing agent used as an additive. The technical details were reported many years ago in the print edition of American Rifleman magazine.
In most situations, the AR-15 gas tube is kept clean by the high temperatures & pressures of the propellant gases going through it. Carbon and other components of fouling normally stay in a gaseous state and are vented out of the rifle before they precipitate out and stick to parts.
The gas system of the M1 Carbine met similar design criteria: it was decided that disassembly and cleaning would be impractical for the individual soldier in the field, so the gas port was moved farther back on the barrel, thus utilizing propellant gases that were much hotter and at higher pressures, minimizing deposition of fouling. Since users could not clean the Carbine’s gas system in detail, the War Dept also required that companies making 30 Carbine ammunition use non-corrosive priming. It became the first US military cartridge to be thus primed. Standard 30-06 rifle ammunition was loaded with corrosive primers all through WW2 and into the early 1950s.
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