If you're building an AR-15, or for parts for replacement or upgrade, a quality gas tube from DPMS is a great option. Select your gas tube based on the location of the gas port on your barrel. For instance, most 16" AR-15 barrels have a carbine-length gas port, and will be sold as a "carbine" barrel. However, if your barrel is sold as a "mid-length" barrel, you will need a mid-length gas tube. Gas tube roll pin sold separately.