I'd go for either the Tikka T3 varmint or a RRA AR15.
The Tikka has a 1:8 barrel twist and the RRA varmint models come with a 1:9 I think.
I have a Tikka T3 and its a beautiful gun, great trigger great action and accurate. I've shot 40gr bullets up to 69gr bullets and both worked good.
I'm loading up some 53gr Hornadys at the moment and should be able to get well under half inch at 100yards.
I've just brought a 4-16x42 Nikon Monarch for $408 from 4scopes who were very good to deal with.
I haven't mounted it yet but from playing with it, it seems excellent.
Clear optics, very positive clicks and the parrallax(side focus) seems to work very good.
It'l be a lot better than my 3-9x40 Burris I have now.