Is this for CMP leg matches, NRA HP service rifle, and/or military service rifle matches?
For CMP leg matches and NRA service rifle class, you need the 'standard' as-issued hand guard design. I'm not sure of whether there are exceptions within the military matches.
The NRA and CMP DR rules are pretty close regarding service rifle specs.
This is the relevant section of the rule book - http://www.odcmp.com/Competitions/Rulebook.pdf
6.2.3 U.S. Rifle, Caliber 5.56mm, M16
The rifle must be an M16A2 or M16A4 rifle issued by the U.S. Armed Forces or
a commercial rifle of the same type and caliber.