Quote History Quoted:
Not in its regular configuration, still has to meet the rules for everything. An "M110" service rifle is nothing like an M110 at all. Caliber and barrel length are about it.
I can't post the rules right now but a quick search of NRA high power rifle rule book will pull it up. It's fairly specific, yet vague.
View Quote
http://competitions.nra.org/documents/pdf/compete/RuleBooks/HPR/hpr-book.pdf - Page 12
U.S. Rifle, Caliber 7.62 mm M-110 series–– In all courses of fire and
in all positions the standard 20-round box magazine or a reduced
capacity magazine of the same external dimensions will be attached.
The flash suppressor may be removed or the rifle may be manufactured
without a flash suppressor. Barrel length may not exceed 20 inches,
as measured to the end of the riflling in the barrel. The front sling
must be attached to the end of the handguard, and must remain in the
6 o’clock position no more than 1/2 inche from its original location.
The sights must be of the standard design found on the M-16 series
of firearms. Rear sight windage and elevation adjustments may be
modified to allow finer adjustments. Plastic covers may be used on
the mounting rails on the handguard
http://thecmp.org/wp-content/uploads/Rulebook.pdf - Page 34
Basically, it has to look like an "unmodified" M16A2 or A4, and have a trigger pull weight of 4.5+lbs.
ETA: NRA is more lenient on their definition of "service rifle" compared to CMP.
ETA2:
http://www.lewismachine.net/media/LMT%20HP%20Rifles%20Final.pdf