I got a chance to briefly shoot a RRA Varmint just the other day, but it was a 24" barrel. The owner had it set up with a bipod and scope (a variable-power set at 14x) as a dedicated paper puncher. As you may already know, the RRA website promises 3/4 MOA with this rifle.
Well, it was near the end of the day and I still had 15 rounds of Black Hills 55 grain FMJ remanufactured ammo (blue box) so I asked if I could shoot it out of my buddy's RRA Varmint to see how it would do. Even though it had a bipod, I kept it folded up and used my front sandbag instead. The first group was 1.5", but if you discount the called flyer, it was 1". Not great, but not horrible either. I knew it was supposed to be a 3/4 MOA rifle, so pride was on the line and the first four shots of my next group measured 3/4", but I threw a flyer again, this time on the last shot :banghead: to make a 1.25 inch group.
The last five rounds I let another friend shoot and he put three of them into a 1/4 inch pyramid
but we won't mention where the other two shots went (hint, we didn't bother to measure the entire group).
Before I shot the rifle, I questioned the wisdom of building an AR into a varmint gun since I figured a bolt-action would be a better platform, and less expensive to boot. Now my opinion is "That's a really cool gun, and that's all the reason anyone needs!"
If this is gonna be a dedicated varmint gun, or if you are gonna be changing uppers, I say "Go for it!"
One note: I don't hunt, but I talked to a friend who does and he said when he built his AR varmint gun, he special ordered the upper with a barrel with
1 in 12" twist rate because that is a better twist rate for the lighter bullets he would use for varmint hunting. However, I've heard the RRA with 1 in 8" twist seems to shoot well with a variety of bullet weights and types. YMMV of course.