The Gulag - Electrified Pool and Assault Rifle

Pick up a relic (7/18), a Trinity Patch from the Watcher in the Woods set, sitting on the floor near the winch.

Now stand near the winch...

...draw your bow, and shoot the rope-wrapped socket dangling from the ceiling into the pool.

Lara automatically shoots a rope arrow into the socket and anchors it to the rope coil wrapped around the winch.

Use your axe as a handle to crank the winch, raising the socket out of the pool. When you've cranked as far as you can, the socket will remain elevated...

...so you can cross the pool without being electrocuted. Break open the salvage crate. Then pry open the metal door on the right.

This triggers a cutscene in which Lara repels a Trinity soldier and steals his ASSAULT RIFLE.

When you regain control, immediately shoot the man lunging toward you.

Then kill his 2 buddies just outside the door.