M4 Inaccuracy Trivia Something I noticed about the Blue Shift Mod weapon for the “M4” is that it’s actually not an M4. This rifle is actually a M16A2, which is the predecessor to the M4 and you can even see it right above the magazine port. M16A2 entered service with the U.S Marine Corps in early to mid 1980s followed by the U.S Army in 1986. The M4 was introduced in 1994 for select Special Forces, but did not enter into larger replacement service for the U.S Army for a decade, and the U.S Marine Corps in 2016. It’s a common misconception as even Gearbox and Valve have labeled it so, since the differences to the rifle series is not as apparent to the common eye, such as the carry handle type for example. Both fire 5.56 rounds rather than the 9mm round of the Half Life MP5 rifle it stands in for.