I agree with all of the above, having built a DCM service rifle to compete with in CMP Service rifle. However, unlike the standard 20" AR15, the DCM rifles have quite a few specialized features like DCM legal free floating handguards, 1/4 MOA rear sights with .036-.040 apertures, match 4.5lb triggers, special air-gauged stainless 1:8 barrels like Krieger and Douglas, and the like. It would be very easy to spend as much on a build as a complete rifle with these components if you are not careful. A true hooded national match rear sight alone can cost $120 just for the sight assembly. There are many imitators out there who are otherwise reputable who sell 'National match" rear sights, which are not true 1/4 MOA increments but instead simply have had the smaller apertures installed on them. BEWARE questional "national match" parts. The advantage of buying a new prebuilt national match rifle is that you are guaranteed to have everything as it should be. Also barrel breakin is critical with these rifles. Armalite and bushmaster's DCM rifles are good examples. If you shoot with the ODCMP for one round with one of an affiliated club's loaner M1 Garands or a borrowed AR15, you will also be qualified to participate in their discount purchase program, which entitles you to a new Bushmaster DCM rifle for under $900, which is regularly about $1300.
If you are in the military, check to see if your unit or a unit at your post has a CMP team, they may be happy to let you shoot with them.
If you do decide to build a true DCM/CMP legal service match rifle, be very careful, or you may end up with something less than what you could have had. Check out [url]www.compasslake.com[/url] (compass lake engineering). Their rifles have won quite a few matches with the right shooters, and they are a good resource whether you build or buy.
Good Luck, see you at Camp Perry!