The M4 Carbine is a family of firearms tracing its lineage back to earlier carbine versions of the M16, all based on the original AR-15 made by ArmaLite. It is a shorter and lighter construction of the M16A2 assault rifle, achieving 80% parts commonality with the M16A2. The M4 has selective incandescence options including semi-automatic and M4 For Sale three-round gush (like the M16A2), while the M4A1 castaway a "full auto" option in field of the three-round burst.
The M4A1 carbine is a differing of the basic M4 carbine intended for definite operations use. The M4A1 is used by almost all U.S special bit units. The M4A1 Carbine is specially favored by counter-terrorist and special operations units for close quarters engagement because of the carbine's compactness and firepower.