44 magnum ammunition out of the prolific 10mm round. If you're using nothing but revolvers I would still recommend carrying it, since you can manufacture. The 3rd best revolver would be the Mysterious Mangum however, the mysterious magnum has very low durability. 357 magnum round, which can be hand loaded out of the other small pistol rounds. The 2nd best revolver in the game would be lucky, which can be found in a level 75 floor safe in the Bison Steve's gift shop.
Otherwise you can just aggravate a ranger holding a Ranger Sequoia to draw on you, shoot it out of their hands, pick it up, and then run like hell.
It's possible pick one off a NCR veteran ranger by reverse-pickpocketing a fully repaired hunting revolver into his/her inventory, then take the Ranger Sequoia once they've equipped the hunting revolver.
