First, ignore post #5 for now.
Either the Rock River or the Armalite is capable of taking you to High Master and getting you a Distinguished Rifleman badge, right out of the box. My rifle is an Armalite, but a Rock River will serve you well also. The only weakness seems to be in the RRA two-stage triggers, which can go down sometimes (losing the second stage is most common, I *think*). Shooting Highpower, your X-ring is 1-MOA at 300 and 600 (1.5 MOA at 200), and I've only met a couple of shooters capable of keeping 98%+ of their rounds in the 10-ring, which is twice as big. Almost any old 69-77gr match bullet will shoot well out of these rifles at 200 and 300. Some people shoot the 75 and 77 grain magazine bullets at 600, but I prefer the 80gr Nosler. Sierras are probably the most popular, but I buy based on price.
Here is the
NRA's definition of a Service Rifle in Highpower, and the trigger weight is 4.5 pounds. Incidently, mine has never been worked on and just makes the check weight at Perry.
So pick whichever one you can get the better price on, get a good sling and go join in! No need to wait to participate, find a new shooter clinic, try it and dive in. You'll learn more, faster, from the clinic and shooting in matches, than you will learn practicing on own. And if you have any questions on anything, just ask. Most of us Highpower shooters are plenty happy to help out new shooters. We're kind of pushers; the first one is ALWAYS free...