The vast majority of AR platform rifles are operated by a direct impingement gas system that cycles the bolt. The design and the difference in this system will affect the shooting experience of your rifle and even its performance in certain circumstances. Check out our blog on gas systems to learn how they work in detail by clicking the picture below. Fine-tuning the recoil of your rifle is also possible with an adjustable gas block that lets you select the amount of gas entering the system. It is important to note that too heavy of a buffer weight or improperly adjusted gas blocks can potentially cause the rifle to cycle unreliably as the gas pressure acting on the bolt carrier group will not be able to move the buffer effectively. For quick reference here is a reference table of gas systems, barrel lengths, and the position of the port. Pistol Length: Barrel Length Less than 10 inches, Position of Gas Port 4 inches from chamber, Carbine Length: Barrel Length 10 to 18 inches, Position of Gas Port 7 inches from chamber, Mid Length: Barrel Length 14 to 20 inches, Position of Gas Port 9 inches from chamber, Mid Length: Over 20 inches, Position of Gas Port 12 inches from chamber.