A3 vs. A4 upper

[erblo]

What is the difference? Is there one?

[ar15barrels]

From Wikipedia: http://en.wikipedia.org/wiki/M16_rifle

M16A3
The M16A3 was a fully-automatic variant of the M16A2 adopted in small numbers around the time of the introduction of the M16A2, primarily by the U.S. Navy for use by SEAL, SeaBee, and Security units.[12] It features a Safe-Semi-Auto (S-1-F) trigger group like that of the M16A1.

Some confusion continues to exist regarding the M16A3. It is often described as the fully-automatic version of the M16A4. Descriptions of the M16A3 that claim that it shares the M16A4's Picatinny rail are incorrect. This misunderstanding most likely stems from the usage of the A2 and A3 designations by Colt and other manufacturers of weapons before the official adoption of the M16A4. Many manufacturers of civilian AR15 rifles designate variants with flat top confirmations as "A3" rather than "A4." Colt used the "A3" designation in the hopes of winning military contracts as they also did with the terms, "M4" and "M5."


M16A4
The M16A4, now standard issue for front-line U.S. Marine Corps and some U.S. Army units, replaces the combination fixed carry handle/rear iron sight with a MIL-STD-1913 Picatinny rail, allowing for the rifle to be equipped with a carry handle and/or most military and consumer scopes or sighting systems.[12] Most of the U.S. Marine Corps' M16A4s are equipped with a Knight's Armament Company M5 RAS handguard, allowing vertical grips, lasers, tactical lights, and other accessories to be attached. In U.S. Army Field Manuals, M16A4s fitted with the RAS are sometimes referred to as M16A4 MWS or Modular Weapon System. This model retains the 3-round burst mode of the M16A2.[12]