This could, very well, belong in Help. However, I'm posting it here because I'm providing the info rather than asking for it.
Everyone knows the RPG is a Soviet- or Russian-built weapon depending on the generation of the weapon. Thus, the acronym is romanized Russian, not English. In Russian RPG is short for Ручной противотанковый гранатомет (Ruchnoĭ protivotankovyĭ granatomet), meaning hand-held anti-tank grenade launcher. However, English-speakers, at least tend to transpose RPG as the acronym for rocket-propelled grenade, which would have been реактивная граната (ryeaktivnaya granata; RNG?).