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The difference between Gas Piston and Direct Impingement technology for an AR-15


ar-15 gas piston
The AR-15 Rifle, while eminently customizable, carries with it a few conundrums. One is the choice of operating system; do you get a model which uses gas piston technology, or the more traditional direct impingement?

Fans of the gas piston will state that impingement models are prone to jamming and often foul easily. Fans of the impingement model will call gas piston tech mechanically unsound. Who is correct, and what is the real difference between the model types here?

Basic AR Functionality – What makes an AR-15 an AR-15?

To qualify as an AR-15, a rifle must be self-loading, and be able to perform a specific set of basic functions mechanically, without user assistance. To be more specific when depressing the trigger, the rifle needs to fire a single cartridge, and then extract that spent case from the chamber and eject it in some manner. Next, it must then load an unspent cartridge into the chamber. The round is plucked from the magazine, the breech is then locked, and the hammer cocked. The rifle will then have a fresh round loaded, and again be ready to fire.

Direct Impingement – How do the original AR-15 models work?

Direct Impingement is the original technology, devised by Eugene Stoner. Propellant gas is bled through a small hole located in the barrel, which is then channeled through a very small tube where it can proceed to directly contact (or impinge) the bolt carrier mechanism. At this point the gas is pushed to the rear of the rifle, and the spent case is extracted and ejected. It is then pushed forward by spring-loaded action, and strips an unspent round from the cartridge, loading it directly into the chamber of the barrel. Contrary to the statements of Gas Piston proponents we have put over 2000 rounds through rifles without cleaning and without malfunction.


Gas Piston Technology – How do the new piston technologies work?

Gas piston technology was first used in modern firearms by Mikhail Kalashnikov on the AK-47. While similar at first blush to direct impingement systems, there are a few key differences in operation. The firing process again begins with propellant gases being bled into the barrel. However, instead of being forced into a tube as it is in a direct impingement system, it is contained in a separate cylinder.

This cylinder contains a piston(similar in operation to what you may find in an AK-47). The gas moves the piston, is in turn pushes the bolt carrier rearward to handle the extraction and ejection process, and then is moved pushed forward to the closed position by a spring just as with direct impingement.




Which system is better?

The direct impingement has proven itself through the years on the AR-15 platform and replacement parts are inexpensive, easy to obtain, and generally made to a set “mil-spec” standard.  Due to the hot gas from the fired cartridge being redirected into the action it will quickly heat up and become dirty requiring a cool down period before the bolt carrier can be removed from the rifle.

The action of a piston rifle remains cool and clean, even after shooting 100 rounds in rapid succession. The bolt carrier can be removed immediately and held in your hand without burning yourself. The trade off for the action staying cool and clean is that you will experience snappier recoil when shooting which makes the piston rifle a little less accurate, especially for follow up shots. Finally, piston system parts are not interchangeable between manufacturers due to there being no set standard and the use of proprietary pistons and bolt carriers.


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I have had both DI and piston rifles. My personal opinion is as range/competition pistons are a better choice. They are cleaner and i personally dont notice the accuracy difference. DI would be my combat choice as the parts are easier to replace, proven in battle. Both are great systems and it boils down to ones preferrence.
Posted @ Friday, September 06, 2013 12:44 PM by Ben C
I have never used a M-16 or AR-15 with a piston system. I would dearly love to try one. During my 24 yrs of active service, it was always a pain to cool down and swab the bloody thing after use.
Posted @ Friday, September 06, 2013 1:17 PM by Retired USAF
I don't notice a difference in accuracy between the two. I shoot mostly kneeling or prone. One thing I do like about the piston is there is less gas that escapes neat the charging handle into the eyes. Typically I rest the tip of my nose against the charging handle to achieve the same sight picture. I only have about 1k through my piston AR; however, I think the build quality and warranty of Stag is great no matter what model you choose.
Posted @ Friday, September 06, 2013 2:21 PM by Warren
What about foreign object reviews. Which works better after being submerged in water, or dropped in sand. I used an A2 in the Mid East and never had a jam, so I can't say. I have a PDS now and absolutely love it. However, neither one have ever let me down.
Posted @ Friday, September 06, 2013 2:47 PM by ROGUE
Pistons guns have actually been around much earlier than the AK-47. M1 Grand and Carbine, FNFAL, M-14 are examples.
Posted @ Friday, September 06, 2013 6:50 PM by John D
It's the old carb vs fuel injection for drag racing. Cost, it's hard to beat a DI gun. Reliability, I've fired 10,000+ rounds from a DI gun and the only malfunctions came from bad mags (ie metal mag worn out, damaged, or smashed), not the firearm. Piston guns run cooler in the BCG and are nice at the end of the day because no to little gas gets on the BCG, but you pay a premium. And as they said, parts are proprietary on piston guns. Whereas, my polymer AR and my brother's forged 7075 T6 home milled lower can share nearly all parts as they are both DI guns. Matter of fact, he built his right after Sandy Hook and couldn't find a BCG anywhere. I pulled mine out of mine so he could test his milled gun. Nearly impossible with a piston gun of different makes. 
One huge problem I can think of between the two, I have heard DI guns will not cycle properly on the first few shots in extreme cold. I haven't tested this but my DI gun worked fine at about 40 Degrees F. 
Lastly, besides price, any AR15 will serve you well. Some say get a piston gun so you don't have to clean as often, but when I asked the gunsmith how often I should clean my gun, he said,"How often do you wipe your ass?" After every time you go to the range, you break a firearm down and clean and lube it. You maintain it religiously, it'll return the favor when your life depends on it. Practice and build muscle memory, and either will work well for you.
Posted @ Wednesday, December 04, 2013 2:14 AM by Joe
Actually John D while I might grant you the FN-FAL which is from '46 (One year before the AK--dunno if that qualifies as much earlier) the others don't qualify. The M-14 as we know it was not seen until the 50's. Though test models that lead to the M-14 were are ound they were not gas piston. The M1 is not piston operated, at least not the models that served in WWII. 
Lastly I will add that the Piston gun just changes where that gas goes, it does not eliminate it. You are going to have to clean that piston cylinder eventually.
Posted @ Wednesday, December 18, 2013 5:37 PM by SH
STG 44 designed 1942 put into service in 1943. Kalishnakov gets all the credit, what about Schmeisser??
Posted @ Monday, January 20, 2014 10:58 PM by Grant Bienert
I think the DI system still gets a bad rap from its 60s era introduction. True you have to clean an AR to keep it functioning well, but this is true with any rifle, even the AK( they can traumatically FAIL as some of our country's enemies have , as well as the semi auto armed backyard bubbas have discovered!) 
I've talked to korean era vets who have seen first hand even the legendary M1 garand jam as well as freeze in adverse conditions, no firearm is flawless
Posted @ Thursday, February 13, 2014 1:16 AM by gonzo1911
Which one is the better to run with a surpressor
Posted @ Wednesday, April 09, 2014 10:17 PM by Sandy Saulter
They both work well with suppressors. With the piston you will want to turn it to the "off" position. The back pressure from the suppressor will still allow it to cycle.
Posted @ Monday, April 14, 2014 2:53 PM by Stag Arms
Brilliant! This is a really marvelous stuff for me. Must agree that you are one of the coolest blogger. was curious to see a stuff like that. Fabulous post!
Posted @ Tuesday, May 20, 2014 4:09 PM by Startups - Starting Business Singapore
Will stag ever offer the piston system for sale as a kit?  
Some of us build for fun and would love the chance to build in other calibers that would do well with the Gas system…Prety Please!!!
Posted @ Tuesday, August 12, 2014 10:44 PM by MiamiNcie57
WE don't plan to offer the piston system as a kit. It is not a drop in type and requires gunsmithing to install.
Posted @ Monday, August 25, 2014 12:20 PM by Stag Arms
Hi and thank you for the responce. Some of us are gunsmithing our builds. Pinning the gas block is not difficult for those of us that have the know how and tools. PS I would like to purchase replacement parts (I.e. bolt / rod springs ) any chance these will be available? Thank you!
Posted @ Monday, August 25, 2014 1:58 PM by MiamiNice57
Is the any difference between the bolts for DI and a GP?
Posted @ Tuesday, September 02, 2014 2:53 PM by Alex
I have been in the Service for over 30 years and can put the below freezing comment to rest on a GI Spec DI AR. I have humped countless miles in the Hindu Kush mountains in the dead of winter (well, well below freezing) and have never had an issue with my M4 or M110. Proper maintenance and temperate appropriate lubricants resolve that issue. On the other hand, I am a certified gunsmith (when the Army gives me the time) and really, really hate to work on piston operated uppers due to the fact that there are no set standards for parts and I wind up having to machine simple things.
Posted @ Thursday, September 04, 2014 7:45 PM by Jimmy Deans
Alex, The bolts are the same for DI and GP. The carriers are different though.
Posted @ Monday, September 08, 2014 1:52 PM by Stag Arms
Is there a difference on the Bolt Carrier Group between DI and piston?
Posted @ Friday, October 03, 2014 3:55 PM by Jerry
Jerry, there is a difference between the bolt carriers. A DI carrier can't be used in a piston rifle and vice versa.
Posted @ Friday, October 10, 2014 2:43 PM by Stag Arms
Thanks for the info. I am considering both DI and GP. Have build / maintenance video from Brownells. 
Posted @ Wednesday, October 29, 2014 6:48 PM by Cliff
I have used the DI system for many years and in live fire combat. Have had some issues due to heat and the action speeding up, very common on full auto. but I have owned both and love the GP system as I don't have to clean it as much and I can't tell the difference in accuracy or notable kick back difference
Posted @ Thursday, March 12, 2015 7:34 PM by A Steele
I have had my 1L for 5 years now, and picked up my 8TL about a year ago. Both great guns; you can't go wrong with Stag! The 8TL runs a LOT cleaner, but I like the weight balance of the 1L better. Neither has ever failed me, but I notice the barrel on the 8TL takes considerably longer to cool down when I shoot two or more mags quickly. No noticeable difference in accuracy when firing with a bipod from the bench. I'd trust my life to either, but would prefer the piston if I had to choose one.
Posted @ Saturday, March 14, 2015 5:26 AM by Ben
Do I need to use a different buffer & buffer spring with a gas piston vs. DI?
Posted @ Tuesday, May 26, 2015 7:48 PM by Courtney
You do not need separate buffers between a piston and a DI.
Posted @ Wednesday, July 08, 2015 1:55 PM by Stag Arms
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