knew something was suspicious about that place, thank you man

Found in the destroyed Doctor's House, on the landmass west of the Sawmill (the same house with the Fireweed growing in front) on the Swamps chapter. It is guarded by two Death Stalkers. The rifle is found on top of the burned-down house, involving a bit of platforming with a few planks that can be climbed leading to it

where did you find that rifle

"We're not worthy!
We're not worthy!"
-Wayne's World