The Colt M4A1 is essentially a
carbinized M16A3. It has an option for automatic fire, and a
Picatinny rail under the top carrying handle(along the top of the
receiver) for mounting various sighting gear, as well as
Rail Interface System(
RIS) on the front handguard for mounting forward tactical grips, grenade launchers, laser sights, etc.
Like the Colt M4, the M4A1 has a shortened barrel and front handguard, and a telescoping carbine-style buttstock. Basically the only difference between the two is the rails and automatic fire on the M4A1, as opposed to the three-round burst of the M4(whose rifle counterpart is the M16A2)
The M4A1 is becoming very popular in the Special Operations community, as it has so much room for expansion. The SOPMOD(Special Operations Peculiar MODification) M4A1 can turn the weapon into any number of various combinations, with visible or invisible laser sights, grenade launchers, reflex sights, even a 4x scope. This weapon will stay in use throughout the world long after it is outphased by guns like the OICW.