There are four main lengths of AR-15 gas systems in use today: Pistol, Carbine, Mid, and Rifle. Each one of these gas systems require different length gas tubes and handguards, so it is important to know which one you have before ordering parts for your AR-15. If you don’t know what size rifle you have, it is fairly easy to determine by simply measuring the length of your handguards or gas tube.
The pistol-length gas system is used almost exclusively on AR-15 pistols or AR-15 rifles that have very short barrels, usually 10 inches in length or less. These systems generally have a gas tube of approximately 4.5 inches. The handguards or free-float tubes for these systems are generally between 3 to 4 inches in length. Some people are now installing low-profile gas blocks and carbine length free-float handguards so that they can have a longer and more comfortable forearm grip area.
The carbine-length gas system, combined with barrel lengths from approximately 10 to 18 inches, is perhaps one of the most common setups across all types of AR-15 rifles. Carbine gas tubes are approximately 7.5 inches long, which means that the handguards will be approximately 6.5 to 7 inches long, such as the Mako Handguard and Rails System, the Daniel Defense Omega Quad Rail, and the Midwest Industries Carbine Forearm.
Mid-length gas systems have become quite popular, and are most commonly used with barrels from 14 to 20 inches in length. They give the shooter a longer sight radius and reduce some of the wear and tear on the upper receiver. Mid-length gas tubes are approximately 9.5 inches and require a handguard around 8.5 to 9 inches in length. Choices for handguards of this length include the UTG Free-Float Mid-Length Quad-Rail and the UTG 2-Piece Quad-Rails.
Rifle-length gas tubes are generally about 13 inches long, requiring handguards ranging from 11.5 to 12 inches long. These are usually found on rifles with barrels 20 inches in length and longer. If you’re looking for replacement handguards for a rifle-length system, check out the UTG Free-Float Rifle Quad-Rail, the UTG 2-Piece Quad-Rails, the AR Carbon Fiber Free-Float Tube, and the Yankee Hill 2-Piece Quad-Rail.