M16A1 still has major use in civilian police agencies, but the US military moved on to the M4A1 back in the 90s. It’s what I trained with, and most of the combat vets I’m friends with were issued M4’s or MK14s, not M16s.
The only National Guard units that are still issued old M16s aren’t combat units, they’re transportation, engineering and other related disciplines. They get the old surplus munitions to use domestically, because it’s not expected they’ll actually need to use them. NG Units deployed overseas at the start of the invasion of Iraq and Afghanistan were issued M4s and other modern rifles.
Edit: The AR-15, which was based on the M16 IS in game, and fills the same role. The actual stats used for the AR-15 match both the military version of the weapon and the M16A2
Edit2: This is more or less correct on current military loadouts for the US