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.

Pistol-Length
The pistol-length gas system is used almost exclusively on AR-15 pistols or AR-15 rifles that have very short barrels, usually 10" in length or less. These systems generally have a gas tube of approximately 4.5". The handguards or free-float tubes for these systems are generally between 3"-4" in length. Some people are now installing low profile gas blocks and carbine length free float quad rail so that they can have a longer and more comfortable forearm grip area.

Carbine-Length
The carbine-length gas system, combined with barrel lengths from approximately 10"-18", is perhaps one of the most common setups across all types of AR-15 rifles. Carbine gas tubes are approximately 7.5" long, which means that the handguards will be approximately 6.5"-7" long, such as the Hogue OverMolded Free Float Forearm, the Yankee Hill 2-piece Quad Rail, the Daniel Defense Omega Quad Rail, and the Midwest Industries Carbine Forearm.

Mid-Length
Mid-length gas systems have become quite popular, and are most commonly used with barrels from 14"-20" 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" and require a handguard around 8.5"-9" in length. Choices for handguards of this length include the UTG Free Float Mid-Length Quad Rail, UTG 2-Piece Quad Rails, Daniel Defense Omega Mid-Length Quad Rail, Hogue Mid-Length Free Float Forearm, and the Troy Industries 9" Extreme Battle Rail.

Rifle-Length
Rifle length gas tubes are generally about 13" long, requiring handguards ranging from 11.5"-12" long. These are usually found on rifles with barrels 20" in length and longer. If you're looking for replacement handguards for a rifle-length system, check out the UTG Free Float Rifle Length Quad Rail, Hogue OverMolded Rifle Length Free Float Forearm, DPMS Carbon Fiber Free-Float Tube, and the Yankee Hill 2-Piece Quad-Rail.