EKM,
I have a nephew who has a Remington Sniper I think it is a SPS in .300 Ultra mag.He has a Sheperd Rang scope 6-18 x40.With 200 gr noslers we were hitting milk jugs at 6 and 700 yds.It was quite simple to use and it was acurte once you figured the wind drift.

Also the Leateher wood scopes look to work or the Springfeild Armory.

As for caliber if I was going that route I would go any fast 300's(Ultra,Warbird or the 30-378)and shoot 200 gr bullets.
BBJ