retraction of my previos statement.
NRA Highpower Rule book:
3.1.2 Service Rifle-U.S. Rifle, Caliber 5.56 min M16 series as issued by
the U.S. Armed Forces, or the same type and caliber of commercially procured
rifle, without bipod or grenade launcher, having not less than a 41/2 pound
trigger pull, with standard-type stock, pistol grip, handguard, and leather or
web sling. The rifle must be so modified as to be incapable of automatic fire
without removing, replacing, or altering parts. In all courses of fire and in all
positions the standard 20-round or 30-round box magazine or a reduced
capacity magazine of the same external dimensions will be attached. The gas
system must be fully operational. External alterations to the barrel, upper and
lower receivers, stock, handguard, or pistol grip will not be allowed, except
that a device may be attached to prevent selector lever movement to the auto
position. The front and rear sights must be the standard design. The rear sight aperture and front sight width may vary in dimensions to suit the shooter. The rear sight aperture may be hooded. Versions or adaptations of the M 1 6 type rifle consisting of external modifications or alterations, will be shot as an NRA Match Rifle, except as allowed in Rule 3.1.3. The trigger weight of these
versions or adaptations will not be less than 4 1/2 pounds, except as noted in
Rule 3.3. 1. A case deflector (DVC-T-30 or commercial equivalent) is allowed.
3.1.3 Service Rifle- Any rifle or modified rifle not covered by NRA Rules
3.1, 3.1.1, or 3.1.2, but permitted by CMP rules are considered service rifles
in NRA sanctioned competition.
later...
3.3 NRA Match Rifle-A center fire rifle with metallic sights and a
magazine capable of holding not less than 5 rounds.
(a) See Rule 3.14, Palm Rest.
(b) A service rifle may be used as a match rifle unless otherwise stated in the program.
Any service rifle used as an NRA Match Rifle shall conform to Rules 3.1, 3. 1. 1,
or 3.1.2 as applies to trigger pull.
(c) A service rifle (Rule 3.1.1 and 3.1.2) whose only modification is to its sights may be used as an NRA Match Rifle. This rifle may use the 20 round box magazine but must comply to Rule 3.1.1 or 3.1.2 trigger pull requirement.
So as I read these rules, you WOULD be allowed to use the 20 round mag as long as the rest of the rifle was SR style and the trigger fit the 4.5 lb requirement. Yes, I believe that the flat top is a sight modification. So you're set if I'm the MD...but some other A-H might say you have more than just the sights modified (carry handle is modified ???) and its his range. Sometimes rules need more clarification, and sometimes they're hazy intentionally.
Personally, I think you're in the clear no matter where you go, but the possibility is there for another interpretation.