Rifle in Vault 15

Solution 1:

It's in the #1 bathroom in your screenshot.

According to this No mutants allowed site, the rifle is

In V.15 in the living section there is also a hunting rifle in the lower right bathroom. once in the bathroom it is in the north west corner.

This is also confirmed by Good Old Games forum

EDIT: Got it. The bathroom in the southwest corner, the rifle is infront on the toilet (i.e. the north west corner of the bathroom. Move the mouse around the area until the rifle lights up. Its a little hard to see - I would have screenshotted it but I was so exicted to have found it I picked it up straight away.