Don't forget the M1 Garand as a Service Rifle. Don't see many of them on the line any more, but it is still a CMP/NRA legal option.
Most competitors are using AR-15's launching 75 gr bullets or heavier, although some guys getting started with 1:9 twist barrels use 68~69 gr BTHP bullets for a "cross the course" load.
The AR-15 is definitely the easiest to shoot, and the cheapest in terms of getting started with a factory rifle that can take you to High Master or Distinguished.
A factory DCM rifle is available from Bushmaster, DPMS, and Rock River with great features like half minute sights, match two stage trigger, and free floated heavy barrel. Expect to pay a premium, but still WAY cheaper than a match grade M1A.
Jimro