Matthew Axelson in Lone Survivor carried a Mk12 Mod1 SPR. G&P do a pretty good one, you'll want the one with the sliding stock rather than the fixed stock. Attachments wise you'll need a 6-9" harris bipod, Leupold Mark 4 scope with a Docter micro red dot mounted on the rear ring, a PEQ2 laser, Knights Armament vertical foregrip, some Knights rail covers and a really good paint job. I priced one up a few months back and using decent quality reproduction accessories it was about £450-500.