The M136 isn't a rocket propelled grenade.
EDIT: Nvm, I got it mixed up with something else. It is infact an RPG.
Well the Propelled Grenade itself is actually translated as Rocket propelled
Anti-Tank Grenade. But NATO decided it was easier to call the RPG. Basically the RPG is similar to the US Bazooka from world war 2 because both warheads fired a hot stream of metal and both are reloadable. Anyway the AT4(aka M136) is single tube sold a single peice of ammunition. Once fired the tube is discarded. So basically it doesn't really matter what they are called because both weapons fire a rocket.
Speaking of the RPG, why did Bohemia decide to put the RPG7 in ARMA 2? Russia has many new versions of the weapon like the RPG-8 and RPG-29. Since Chernarus was part of the USSR I'm sure the CDF would have a bunch stocked up.