From WikiGTA - The Complete Grand Theft Auto Walkthrough
|GTA Vice City
GTA San Andreas
Liberty City Stories
Grand Theft Auto IV
The M4 or Model 4 Carbine is an improved version of the M16 (which is available in GTA III). The M4 is a very powerfull assault rifle, with an even larger range and higher firing rate than the AK 47. The high firing rate makes it ideal for attacking both charging and fleeing enemies, but its recoil makes it impossible to shoot while you're walking. In GTA Vice City it's possible to switch to FPV so you can aim more accurately.
In GTA San Andreas, You reach the 'gangster level' at 20% weapon skill, and 'hitman level' at 100%.