The M40 is a Poor-condition bolt-action hunting rifle used as an early sniper. Its magazine is broken, so it loads one round at a time. It hits hard and reaches far, but you will want a better rifle soon.
Use it to snipe lookouts and drop single humans before a camp fight. Missing a shot costs you a full reload, so take your time.
The C8 Rifle is semi-automatic with 5 rounds. The Talon 7 and .50 BFG come later and hit far harder. The M40 is only worth carrying early on.
.50 BFG
The .50 BFG is a Special Forces sniper rifle sold at Wizard Island from Trust level 2. It hits harder than almost anything in the game and holds 10 rounds. Bring it for Reachers, Ragers and anything you want dead in one shot.
BND-150
The BND-150 is an experimental IPCA sniper rifle that exists only in New Game Plus. It unlocks once you finish the main story again in that mode. It fires 6mm rounds and three craftable dart types, which makes it the most flexible special weapon.
C8 Rifle
The C8 Rifle is an Average-condition semi-automatic sniper rifle for the special slot. It lets you take precise shots from far away. The Lost Lake merchant sells a magazine upgrade that takes it from 5 to 7 rounds.
SSR
The SSR is a semi-automatic sniper rifle with a large magazine and no delay between shots. You get it as a reward near the end of the main story. It suits hectic fights where a bolt-action would be too slow.