There is no guarantee that you'll find a rifle at every given location, but there do seem to be designated bow, rifle and revolver spawns in the game. Like you said, there's no guarantee that if you go to signal hill there'll always be a revolver on the bed, but I just found one there in my game. So actually all weapons(including knives, hatchets, and other things) do have designated spawns, where you're not going to find them 100% of the time, but rifles don't turn up in a cabinet in a coastal house, either.