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.
Start New Game Plus after beating the game once on any difficulty. Then finish the main storyline again to unlock it in the Gun Locker.
Normal 6mm rounds handle long-range kills. EDT darts stun, gas darts poison groups, and explosive darts hit packs or big targets. That gives you an answer for most fights from one slot.
Craft one of each dart and get a kill with every ammo type for Logan's Shadow. The darts use cheap parts, so keep a few of each on hand.
.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.
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.
M40
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.
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.