Long range? I personally recommend something chambered for .308 Win (aka 7.62x51 NATO). The AR-10 clones, Springfield M1A, and various flavors of FAL are the most common semi-auto platforms for that caliber. In bolt action your options are even broader, since practically every hunting rifle manufacturer makes something in .308. I suggest this round because it is a "real" rifle round, and in particular because it is so thoroughly ubiquitous.