The M1891 sniper rifle is described in the game as:
A polymer update to the 19th century Soviet rifle.
You’ll find this in central La Cruz province, on a hilltop by a makeshift airstrip. A “Food Air Transport” side mission is nearby to steal a plane. There are five sicarios here, guarding the plane. The weapons case is out in the open at the end of the airstrip away from the plane.
There’s no road to the airstrip, so you can approach this how you like. There should be one enemy alone near the weapons case end of the airstrip you can take out, leaving the other four for a sync shot.
This sniper rifle is nuts. I don’t know why they added this to the game, but it does sound good. Maybe they added it because it’s the only other gun that shares a magazine with the Dragunov (found in Villa Verde). Unlike the semi-auto Dragunov though, this is bolt-action. It doesn’t accept many accessories though. You can’t even suppress it, so it’s always loud as hell. But you can pop a long barrel on it or change the scope.
It has middling damage and penetration, but it’s definitely bested by the more modern rifles. Being bolt action, the standard 10 round magazine should be enough. But if you need it, a 20 round extended magazine is available (in Flor de Oro). Overall, I wouldn’t recommend this gun, but it is easy to collect if you’re in the area.