AR15 Piston vs Direct Impingement: A Debate 50 Years in the Making








Today we will discuss and evaluate the differences between the piston and direct gas impingement AR15 type rifles. Every shooter has different requirements their rifle must meet to accomplish whatever mission they are faced with. We will mainly be focusing on the principles of operating systems and will not be focusing on particular brands. Instead of picking a winner we will start off with this statement: Direct Impingement and Piston AR15s are in my (humble) opinion equally reliable systems. Any gun will do if you will do. This article will give unbiased facts about both systems in the hopes it will help you make the best decision in choosing the best weapon for you and your needs.     


Problems with Pistons (in no particular order):



  • Carrier Tilt: When a piston impacts a bolt carrier group downward force is applied on it. This will cause the carrier to “bounce” inside the upper receiver possibly damaging the buffer tube, receiver extension threads or buffer detent.




  • Weight: Ounces make pounds, pounds make pain. Piston gas blocks are usually several times heavier than typical direct impingement gas blocks. Quality drive rods should be solid steel and are heavier than gas tubes.




  • Added Parts: In addition to the proprietary bolt carrier and bolt; a piston assembly has about 6 additional parts compared to direct impingement.


  • Handguard Compatibility: Because the piston system is not as slim as the direct impingement, handguards that will work with a piston are limited. Many factory piston uppers are higher profile than the mil-spec to accommodate piston assemblies.     





  • Proprietary Parts: This may be the biggest drawback to the piston AR15 type rifle. Every company has a different, patented, piston assembly system. There is no standardization of parts or “mil-spec”. If a replacement part is needed the company that produced it will most likely be the only source. This is problematic if you are in the field and need a new part (odds are the local gun store won’t have what you need). In addition; if the company who produced your piston gun ceases to produce your model or goes out of business you will need to replace your upper if you wear out a part.  

Where Pistons Shine:



  • Cleaner/Cooler Running System: The piston system keeps expanding gasses out of the upper receiver. This will reduce carbon buildup in the bolt carrier group as well as keep temperatures near the shooter’s face lower than that of the direct impingement system.




  • Higher Reliability in Water: The reliability of the piston system in water environments is one reason that drove the United States Marine Corps to adopt the piston operated M27 IAR. Direct impingement rifles can fill with water when submerged and if fired without draining the gas tube the rifle will probably explode. Piston rifles don’t have this problem.  



Summary of the AR15 piston platform: it is a reliable system if measures are taken to reduce wear (such as a buffer tube that prevents carrier tilt and a reinforced buffer detent). Maintenance must still be performed on any weapon. Although the piston guns run cleaner in the chamber and upper receiver areas the bolt carrier group should still be disassembled periodically and inspected. Carbon buildup at the gas block and piston is an issue that must also be addressed, a fouled piston can cause the weapon to malfunction. Be advised: to properly clean and maintain a piston assembly the handguard will most likely need to be removed and zero with iron sights or rail mounted laser systems must be reconfirmed.  If you do not mind the added weight or proprietary parts or you require your weapon to work in a wet environment (beaches, swamps, exc.) then a piston driven AR15 may be for you.


Issues with Direct Impingement:


  • Cleanliness: The DI AR15 uses gas to move the bolt carrier to the rear and cycle the action. There is no way to avoid carbon build up around the bolt and in the chamber. This is made worse when using “dirty” ammo or a suppressor. The weapon will need to be cleaned and maintained regularly.


  • Gas Blowback: With the hot gasses coming back to cycle the action some gas will come back into the shooter’s face. This issue can be reduced by a charging handle designed to block the gas but can never be eliminated.


  • Gas Keys: There are more bad AR15 components out there than good ones. One thing to be aware of when buying a bolt carrier group is the “gas key”. Gas keys must be properly screwed down with proper grade steel bolts and be staked well to keep the screws from backing out. A loose gas key will cause the weapon to malfunction or worse.



  • Gas Rings: Keep an eye on gas rings as they can wear out over time. Replacements are only a few dollars, it would be wise to keep a spare set or two.




  • Lubrication: All firearms need lubrication. The direct impingement AR15 needs more than its piston cousin.  



Direct Impingement Advantages:



  • Parts Availability: The standard civilian AR15 and military M4/M16 use a direct impingement system. There are cross-compatible parts available from hundreds of manufacturers. Gun Repair and Sales on Range 7 has all the essential quality parts you may need for the DI AR15. The majority of parts and accessories today are being made for direct impingement rifles.




  • Lightweight and Slender: With no piston assembly a DI upper can be slim and lightweight. This is one reason why US Special Operations Command chose a direct impingement upper for service overseas.

Summary of the direct impingement AR15 platform: it is a battle proven system (that has served in every environment across the globe for over 50 years) that requires maintenance over time and in adverse conditions. Your equipment is only as good as the care you put into it. If you properly maintain your DI rifle between missions, it will not fail you. Cleaning a direct impingement bolt carrier group will result in dirtier hands and a few extra rags compared to a piston gun. Almost every gun store in America has parts that will work in a DI rifle (even in states that aren’t too friendly to 2nd amendment rights). If you want a rifle that has proven itself, then a good quality direct impingement AR15 may be what you need.


By J.L



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