"DCM"-style rifles generally have two-stage match triggers, free-floated handguards, 1/4 MOA rear sight with narrower front sight post, and a heavy barrel with 1x8 (or faster) twist rate. Some manufacturers like Bushmaster match the bolt to the barrel extension for the most precise lockup possible. Lead weight can also be added to the buttstock trapdoor compartment to acheive a desired balance for the rifle. If you are looking to perform any of these mods to a standard HBAR you already own, the trigger is the easiest to perform and will probably yield the greatest improvement. I'd look at sights next, followed by the free-float handguards. If you're just starting out, I wouldn't worry about replacing the barrel unless you're trying to stabilize heavy bullets at very long ranges (assuming you have a 1x9 twist rate which is pretty much standard). If you don't yet own a rifle, look at the DCM model from Rock River Arms. Lot of bang for the buck, and it's basically ready to compete right out of the box.
Thanks Delta Heavy for the reply, I'm pretty new to the competition so I'm trying to assemble as much info as I can. I'm going to end up building this one, and decided to use the AR rather than the M1A since it seems the AR's are performing better right now. What competition is the rifle your describing used for. I know for Service Rifle it has to be as issued. Is there any thing else you can suggest.Thanks again