The marksman carbine guns in Fallout New Vegas are well-suited to the game's sniper-friendly gameplay, and the All-American rare / unique weapon version is devastating. It's also hard to find, so watch this video to learn how to locate it.
From the Creator:
Located in the armory of Vault 34. On the floor in the Armory cache, near the overturned table. If Boone, Cass, Veronica or Raul are in your party, and you can't seem to find it, check their inventory as they may have picked it up.