I know this is real steel info but it will give ya some good ideas.
http://en.wikipedia.org/wiki/Designated_marksman
Personally I use an M14 for this type of job. When even lightly upgraded you get some good distance with it amd when you put a scope on it, it just looks pimpin. I cant wait till I get the rest of my parts in to create the Smith Enterprise M14SE "Crazy Horse" look for my M14