@RustyCrab in mystery lake there's a high probability of finding vaughn's rifle in or around the dam
I often find a revolver at the ruined lookout next to a dead body on the ruined tower stairs (the fire tower that is wrecked, on the peak with the radio tower, across from the peak with the good fire tower)
check the good fire tower too, you can find a revolver or rifle there but that's a lower probability spawn in my experience. usually some other good stuff there anyway
trapper's homestead obviously but I almost never find a gun there. usually some ammo
then there's a prepper cache that's kinda hard to find. basically go across the lake to the rope at the far right corner, go up, go along the ridgeline until you're looking down at the tracks that go to forlorn muskeg, then go along the ridge to the right and the prepper cache is on a little flat spot where you can't see it until you're right on top of it. only a 1/9 chance it'll be the gun cache, 1/3 chance it will be a stocked cache of some kind