Question About U.S. Military Arms

[Kinzer]

I've always wondered how the names of weapons used by the military are decided on and what they mean exactly? M16, M60, M9 etc. What do the numbers mean. With the M1911, it's pretty clear, with everything else not so much. Also, some have the same name it appears. There's an M2 flamethrower and an M2 mortar.