AR-15s usually come with basic plastic two-piece handguards. One of the more popular upgrades done is to replace these handguards with a quad rail which allows you to mount accessories such as lasers, bipods, and flashlights.
Two-Piece Rails vs. Free-Float Rails
Two-piece rails are the easiest to install. You remove the plastic handguards by pushing the delta ring towards the receiver, and lifting off the pieces. Sounds easy, but the spring behind the delta ring is strong. You will need 3 hands, a friend, or a handguard removal tool such as ARR-046. Examples of two-piece rails are ARR-238, ARR-146 and ARR-148. The new rails are a direct replacement for the plastic handguards.
Free-float rail systems require more work. They are usually one complete piece that mounts to a replacement barrel nut. No part of the rail touches the barrel. This can aid in cooling and accuracy. You will usually need to remove the flash suppressor, front sight post or gas block, gas tube, delta ring assembly and barrel nut. This would be a great time to replace any part of your upper assembly like the barrel, flash suppressor, gas block, and front sight, since you would be removing everything. Tools required to install a free-float system include a vice, upper receiver action block, small hammer, punches, small pliers, hex wrenches, torque wrench and a good AR-15 Multi Tool. If you acquire all of the tools necessary to install a free-float rail system you will then have all of the tools necessary to build your own upper from scratch. Be prepared to have all of your shooting buddies bringing you their rifles to install their rail systems. It is easy to do with the proper tools.
What Size Do I Need?
For the most part AR-15 uppers have three different sizes; carbine, mid-length, or rifle length. Rifle length handguards fit on a 20” barrel. Carbine lengths are 16” or less, and mid-length are usually found on 16” to 18” barreled uppers, but use a mid-length gas system. Rifle-length rails are 12” long, mid-length are 8-1/2 and carbines take 6-3/4 rails.
Exceptions are Colt carbines, which are 6-7/8” long.
Rail Covers- Covers vs. Ladders
Once the rails are installed, they look great, but are not comfortable in your hand. Many AR-15 owners use rail covers or ladders to protect the rails and their hands when shooting. Covers completely cover the rails and offer the most protection. Ladders fill in the portion between the rails, providing some comfort and protection and are not quite as bulky as covers.