HA HA M-16s aren't "Machine Guns" they are Assualt Rifles. And the M203 is not the most commonly attached equipment. That would be the M68 Sight and the PAQ-4 Infaread targeting device or the PEQ-2 Infared targeting device that EVERY member of my squad has as opposed to the 2 guys in the squad that have M-203 Grenade Launchers.

Lighter virtion of the m-16

The M-4 is a version of the M-16 machine gun with a 16" heavy barrel which has a special machined groove which is used inconjunction with the bayonet lug, to allow attachment of other devices. The most commonly attached device is a grenade launcher.
This groove is midway between the front sight and the flash-hider on the end of the barrel.