When we had the M67 90mm Recoilless Rifle, we use to call it a "90." We also use to call it the "old stove pipe."
We would call it an "84," but when it first came into service we already had an 84mm anti-tank weapon (AT-4, the M136), which BTW was made by the same company that makes the M3 Carl Gustav. Also, at first we didn't even know the nomenclature (M3), so we just called it the Gustav. They [command] first started calling it the RAAWS (Ranger Anti-armor Anti-personel Weapon System), but we thought that name was gay. I see now it is called a MAAWS (Multi-Role Anti-armor Anti-personnel Weapon System)... Still gay.