The M1 rifle (also known as the M1 Garand) is a different matter. It shoots a full power cartridge--the 30-06--and is quite a bit heftier. It uses an eight round en bloc clip.
The M1A is Springfield Armory's designation for their semi-auto version of the M14. It has a detachable box magazine and fires the 7.62x51 NATO cartridge, which is shorter and perhaps 5 to 10% less powerful that the 30-06.
My bad, I was thinking Garand vs. Carbine. :)