Zombie Gunsmith
You're on a ridge. Eight hundred metres out, a laboratory is burning — the place the outbreak started. They're coming out of the hole in the containment wall and walking straight at you. You have a Barrett .50 cal, a scope that goes to 16x, and as long as it takes to line up the shot.
Build the rifle first: optic, barrel, muzzle brake, bipod, ammunition. Every good part costs you something. Then hold your breath and start picking them off. The further out you drop one, the more it's worth.