Good question, glad you asked. When the military adopts a new weapon platform, especially in cases like the M16 where the differences were minor in subsequent variants, it is the frontline guys who get the shiny new toys. As for us "rear" guys (I was assigned to an artillery unit) and for training units such as basic training, the older variants would still be used because it would have jacked up the bill significantly to replace all guns at once.
In many units, at least one M203 grenade launcher would be assigned per section/squad. Guess who got use that boop toob?
