I'm assuming because it isnt bolt action and you need to fire pretty fast in some of the events from what I can see. That rules out the remington 700 and Styre mannlicher 308. Sorry for all the questions, just trying to get a feel for the sport before I jump in.
Actually, the AR15 is the most popular for the simple reasons that it is the most economical AND most accurate of the three SERVICE rifles currently approved for competition (the others being the M1, M14, and their civilian clones).
NRA Highpower has two rifle divisions: match rifle and service rifle. Service Rifle is the three rifles I mentioned with unlimited INTERNAL modifications (except for the caliber itself) and very limited EXTERNAL modifications that are specifically enumerated in the rule book.
Match Rifle is almost anything goes so long as it has metallic sights and can hold at least five rounds in the magazine.
CMP Excellence In Competition Matches are a special form of Highpower open only to competitors with Service Rifles.
Do not be misled to think that bolt action rifles are uncompetitive in Highpower because two of the stages are rapid fire. Quite the contrary, the time limits for sitting and prone rapid fire are more than adequate to clean (all 10s) the target with a bolt action target rifle. In fact ALL NRA national records for sitting and prone rapid fire are owned by shooters using bolt action rifles. In fact, Chief Warrant Officer 4, USMC (Ret.) Dennis De Mille OWNS the sitting rapid fire national record with an incredible 300-28X. He did it using a TUBB 2000 bolt action match rifle.
Rapid fire is actually a misnomer. The stages should really be called sustained fire. Most new shooters hear "rapid fire" and try to blast their ten rounds and quickly as possible, ending with a dismal score, while those who have developed a pace that uses about 90% of the allotted time end up with scores in the 99% to 100% range regardless of the rifle used.
The AR15 is popular not only because of reasons of cost and accuracy, but because most shooters prefer to shoot service rifle for a variety of reasons.
I started in the sport with one and made it to Master, but my eyes have had increasing trouble focusing on the front sight post as close as it is to the eye. So I switched to bolt action match rifles and I am much happier. I haven't shot any Conventional Highpower this year since I am concetrating on Highpower Mid Range Prone but last year the few times I took my Model 70 out I easily matched my AR 15 scores in sitting rapid fire and shot better in prone rapid fire.