What is the difference between a M16a4 and an M16a2?

To begin with, the M16A2 and M16A4 are very similar weapons, sharing the same trigger groups as well as the improvements made from the M16A1. The differences are subtle. Most notably, the carrying handle is now removable to reveal a 20mm accessory rail on the top of the receiver for easier mounting of optics. A great deal of M16A4s are also issued with a Knight's Armament Company M5 RAS (rail attachment system) handguard, as opposed to the ribbed polymer handguard found on the M16A2 and M16A3. Colt also produces an R0901 variant of the M16A4 for international sale that differs in that it replaces the three-round burst fire mode with full auto.