Why a Piston : A Comparison Review

Why a Piston : A Comparison Review

There is a huge divide in the AR community on the pros and cons between gas piston AR’s and the standard direct impingement systems as used by the US military and by many standard AR’s. This divide is almost as great as the one between ARs and AKs. I by no means am going to tackle all the issues, nor am I making any attempt to prove one system is better than the other. Both systems are very good and reliable. I am simply going to state the pros and cons between the two, and my opinions of each. How Direct Impingement Works I’ll start with the direct impingement system, as it is the most common on the market. This is the standard operating system in most AR’s on the market as well as standard issue for the US military. Here is the basics of how it works. You have a hollow straw that follows beside the barrel starting at the gas block in which the front sight post is attached (or fixed front sight post weapons). This straw goes from the gas block into the upper receiver and into the bolt carrier group (bcg) via the gas key or gas port on the bcg. As a bullet passes down the barrel, some of the hot expanding gases that propel it are siphoned off via the gas block, which then sends them down the gas tube striking the bcg via the gas key. These hot expanding gases then force the bcg rearward, ejecting the spent casing. The buffer spring absorbs much of this and forces the bcg forward once again, stripping the next round from the magazine and chambering. How Piston Systems work The piston system replaces the gas tube with a spring loaded solid rod. This rod attaches to the gas block on the front of the rifle via a small piston that slides in and out of the gas block, and hits a flattened block on the bcg where the gas key would be located. When the round passes thru the barrel, and the gasses siphoned off, they enter the piston chamber, which then pushes the rod toward the bcg. When the piston has moved as far as required, a small vent opens up allowing the excess gases to escape near the gas block. The piston operating rod then strikes the bcg, cycling the weapon. Both weapons generally come standard with a forward assist and dust cover. Let’s briefly discuss these for a moment. First, their uses. Both are items that are highly valued and needed in combat conditions. The dust cover prevents loose debris from entering the bcg while the weapon is in transit (i.e. while on patrol or just hiking from your vehicle to your hunting location). Any loose debris that accumulates within the bcg can cause weapon fouling and malfunction. Under combat conditions, this can be very bad obviously. The forward assist is the push button on the right hand side of the weapon. What this is designed for is if the bcg does not seat all the way forward, preventing the round to chamber properly and the locking lugs on the bcg to engage, you can manually seat it by tapping the forward assist. Once again, in combat conditions, this can be useful from a piece of debris has cause the bcg to foul and not seat. Now for the pros and cons. The gas piston system is usually anywhere from 8 ounces to 1 pound heavier than the conventional direct impingement systems, throwing the front of your rifle off balance if you are not used to it. The direct impingement system is alot dirtier and requires massive amounts of cleaning, oiling, and maintenance. You can imagine how dirty the bcg, upper receiver, and lower must get with all the hot expanding gases traveling directly into it. These gases also cause the bcg to heat up, expand, and “cook off”, if you will, most of the oil on the bcg. Most Piston ARs have proprietary parts, meaning they are not interchangeable if they need replacing, making them difficult to find, while any direct impingement AR parts will interchange with other direct impingement AR. If you find find yourself where you have to cross a river, and have to submerge your rifle and need to immediately fire, a piston won’t blow up on you while a direct impingement will, due to the gas tube filling full of water and needing a few seconds to drain prior to firing.

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