http://www.at3tactical.com/blogs/news/92...as-systems
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.
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.