DI vs Piston
Direct Impingement vs Gas Piston
Direct Impingement and Gas Piston are two methods of gas-operation that provide energy to operate auto loading firearms. In this case, the AR-15 platform. In both systems, gasses from the firing of the cartridge are harnessed through a port in the barrel and are used to operate the action. The AR-15 was initially designed to use a Direct Impingement gas system and was later adapted by other companies to utilize the Gas Piston system.
How Direct Impingement AR-15s work
When the trigger is pulled, the hammer drops on the firing pin setting in motion the ignition of the cartridge in the chamber. The primer ignites the propellant in the cartridge sending the bullet forward. Excess gas from this chain of events is sent through a small hole in the barrel, through the gas block or front sight and returned through the gas tube to the upper receiver.
Once inside the upper receiver, the gas goes through the gas key and into the bolt. As the gas pressure builds, the pressure increases causes the bolt carrier to move rearward, unlocking the bolt. The bolt carrier group is then driven rearward, pushing on the buffer which in turn compresses the action spring.
How Gas Piston Systems Work
Just like a direct impingement system, when the hammer drops on the firing pin the cartridge in the chamber is ignited. The bullet travels down the barrel and excess gas is sent through a small hole in the barrel. In the piston system when the gas exits the hole in the barrel it pushes on a piston instead of traveling into the upper receiver. The pistons movement then causes the bolt carrier group to operate, instead of the gas directly.
There are two styles of gas piston systems short-stroke and long-stroke piston systems. In a short-stroke piston system, the piston strikes a stationary rod like a baseball bat striking a ball sending the carrier rearward. In a long-stroke piston system the piston is attached to the operation rod in some manner and the gas pushes the bolt carrier group and operation rod the entire way.
Which is better?
There is no clear overall winner, each has their own strengths and weaknesses. Either one is an excellent choice for a firearm as long as it is using quality parts. Piston supporters tend to cite cooler and cleaner operation as the main reason to support the piston system’s claim as rightful ruler. Supporters of direct impingement usually cite carrier tilt and accuracy as their main arguments as to why direct impingement is the better operation.
As long as you are purchasing quality parts or a quality rifle for a direct impingement gun chances are the firearm will outlast you. They do require more cleaning and will be subject to fouling and stoppages if they are not kept lubed and in good working order.
One of the biggest things to worry about with a piston system is carrier tilt. It will not kill your firearm in a short period of time but if the carrier tilts excessively in operation it will require the replacement of your receiver extension and possibly your charging handle and upper receiver. The reason carrier tilt exists is because the operating rod is striking the top of the carrier, it is just physics after that.