CMP/NRA highpower Service Rifles have to look like an issue M16. No alteration may change the external configuration of the rifle. Internal modifications may be made to improve functioning and accuracy. There are some rack matches where you are issued a rack grade M16A2, are given some time to zero, and then you shoot a match.
The rifle ust be an M16A2 or A4 rifle or a commercial rifle of the same type and caliber. Must be chambered for 5.56. No automatic fire. Must have a fully operational gas system. Must have standard 20 or 30 round magazines, can use a single load follower.
The rear sight aperture can't have a hood longer than 0.70" and can have a corrective lens in the rear sight hood. Rear sight ele/wind can be modified for finer adjustments. Front sight may be fixed or lockable. Rear sight may be pinned. Front sight dimensions may vary to your liking.
The regular Service Rifle has a 20" match grade barrel either SS or ChromMoly(non-chromed). It can have a flash suppressor and bayo lugor not.
Upper receivers can't be changed during the match. Float tube is used and hides under the handguards with the front sling swivel in the 6 o'clock position near the original position. M16A1 or A2 Buttstocks can be used.
Removeable carry handles and flattop upper are allowed. Probably won't have enough elevation for 600 yds unless you have it worked over.
Match trigger, no less than 4.5 lbs pull wt.
No restrictions on ammunition.
No sling cuffs, just a regular service rifle sling i.e. M1907 or nylon sling
NRA highpower SR and CMP are very alike. Can shoot only in 3 CMP matches plus the one at Perry. No sighters in CMP. Best way to practice for CMP is to shoot NRA.
The CMP and NRA rulebooks are invaluable. Take whatever you have to the local club. Shoot, have fun, listen, and learn. These guys are a wealth of knowledge and can point you in the right direction.