Military rifle designations

[Gatorade]

So is there a reasoning to the designations the military give their rifles? M1, M1A Carbine, M14, M16, M4. I thought I had it down till the M4 came iut. For a long time I also thought the Mini14 was the civilian M14. Also are there any M designated firearms that are available to civilians in their military set up and condition? I know the M1 Garand can be purchased but not the M4 as it is select fire. I think the M14 used by the military is also select fire.

Thanks for the comments!