Highpower Rifle is centerfire, iron-sights only (except in the occasional F-Class event)
There are two categories in NRA Highpower events--Service Rifle (M1, M1A/M14 or AR15 which can be modified internally--the AR is most popular these days) and Match Rifle. There is usually a pretty even split between match rifles and service rifles and they are all on the line at the same time. The CMP (Civilian Marksmanship Program) has their own set of rules and in their matches, it's Service Rifle only. Both matches are otherwise pretty similar.
An Across the Course match is either 50rds or 80rds. The first stage is slowfire offhand (standing) at 200yds. One round per minute (either 10 or 20 total depending on which course of fire).
The second stage is sitting rapidfire at 200yds. You begin standing, and then drop into position when the targets appear. You have 60 seconds to shoot 2rds, reload, then 8rds (or 5 and 5 if you shoot a bolt gun). During an 80 round match, you shoot 2 strings of each rapid.
Third is prone rapidfire at 300yds. Again, you must begin standing and you have 70 seconds to shoot 10 including the reload.
The final stage is slowfire prone at 600yds. 22rds in 22 minutes.
NRA Longrange Highpower is all slowfire prone at 600, 700, 800 and 1000yds if I remember correctly.
F-Class is a NRA rule which if in play allows any rifle, any sight (bipods, etc). You can usually shoot and F-class rifle in most matches as long as there's room on the line, but they won't count your score.
The NRA has shooter classifications--Sharpshooter, Marksman, Expert, Master and High Master. At CMP matches, they award medals and points to the top shooters and when you accumulate enough points you are awarded the Distinguished Rifleman badge (really a military badge). There have only been something like 1800 awarded since the badge was authorized after the Civil War.