Evverything you always Wanted to Know about the M16 series but were Afraid to Ask
The M4 differs from the M16 in these ways:
1. Barrel Length 14.5" vs. 20" Barrel contour is the same as the M16A2 (heavy in front of the handguards) but with a section that has been turned down to the under handguard diameter to facilitate mounting an M203 Grenade Launcher.
2. Buttstock is a 4 position collapsible type.
Those are the primary differences. The M4A1 uses the same trigger group as the M16, M16A1 and M16A3(safe-semi-auto). The M4 uses the same trigger group found in the M16A2 and M16A4 (safe-semi-burst). All currently produced M4s and M4A1s have removeable carry handles with rear sights that are only adjustable to 600 meters for elevation vs. 800 meters for the fixed carry handle versions. Early M4s had fixed carry handles.
Right now the U.S. Armed Forces currently fields the following versions of the AR15/M16:
M16-Original model as adopted in 1963. 20" 1/12 twist barrel, open 3 prong flash hider, locking sights that require a tool (nail, ballpoint pen, bullet tip or even sight tool
) to adjust, triangular handguards, original length buttstock without a compartment for cleaning supplies, no forward assist. Many still in service in the USAF.
M16A1-Same as the M16 but with closed "birdcage" flash hider, trapdoor buttstock and forward assist. Thousands still in service with Army reserve components.
M16A2-Adopted as standard by the USMC in about 1982 and the Army in 1985. 20" barrel that is heavy in front of the handguards in 1/7" twist. Flash hider closed on the bottom to reduce dust signature when firing from the prone. Front sight post changed from round to square. Handguards changed from triangular to round. "Click" adjustable rear sight, windage and elevation to 800 meters. Lower receiver reinforced near receiver extension tube. Buttstock made 5/8 inch longer and buttplate changed to make it "stick" in the shoulder pocket better. Finger groove added to pistol grip. Fire control system changed from safe-semi-auto to safe-semi-burst, the burst position being a cam device that will fire one to three rounds per squeeze of the trigger depending on where the cam was when the trigger was released from the previous burst. Standard issue in the USMC and regular Army.
M16A3-Same as the M16A2 only has a safe-semi-fire trigger group. Found in the Navy
M16A4-Same as the M16A2 only has a removeable carry handle with 600 meter rear sight. Most of them have the M5 Rail Adapter system in place of the standard handguards. Found in Army Special Operations Units.
M4-Carbine version of the M16A2. Differences as noted above. Found in Airborne, Air Assault and Light Infantry units. has replaced the M16A2 in most of these units. Also issed to replace the M3 "Grease Gun" as BII on armored vehicles.
M4A1-As noted above, found in Army Special Operations units.
There may be minor differences in the way these weapons are fielded but that's basically the down and dirty of it. I understand the Marines have a limited number of M4s and the USAF has converted some M16s to M4s and armed PJs and CCTs with them.
HTH
Jeff