Im about to order a set of free-floating handguards for my AR. There are lots of aluminum and fiberglass free float tubes that say they are "national match".  I though to stay in the stock service rifle competition your gun had to look just like a regular M-16, but those completely round and aluminum tubes don't do that. What do I have to get to stay in service rifle competition. Im just shooting for fun, but would like to be in the same category as everyone else.

None of the aluminum tube "race gun" float tubes are DCM legal. Your rifle MUST look like it has standard hand guards on it. Try Compass Lake Engineering... that's what I put on mine. Your grip... same story... has to be a stock grip. I put a Hogue on mine because, so far, Ihaven't shot any DCM matchs and the Hogue is NRA High Power legal. If I decide to shoot a DCM, the original grip is in my range box and it takes 2 minutes to change if I take my time.

What he said.  The handguards must look like regular round handguards, or you'll be shooting a match rifle.