What does the M mean in M14 M16 M4 etc?

It could have been as the US Navy does it: BB for battleship; DD for destroyer; CA for Heavy Cruiser; CL for Light Cruiser; DE for Destroyer Escort; or CV for aircraft carrier (C-Carrier, V-Heavier than air). Instead of MM-14, MM-16, or MM-4 it's just M-14 or M-48 Patton tank...the "M" stands for "Military Model."