So i've spent quite a bit of time exploring mystery lake and the rifle spots are pretty consistent if you know where to look. The forestry lookout and trapper's homestead are your best bets, usually on their gun racks - i always check these first. There's this spot called max's last stand near trapper's that sometimes has one by a frozen body, and while you're out there keep an eye on the lake trail too cause i've found rifles near corpses especially when you see crows circling.

The destroyed lookout can be worth checking, and theres this neat little cabin called lone lake where i've spotted rifles leaning against the outside wall near where the bear hangs out. Pretty spooky at night but worth the trip! If you're lucky enough to find a prepper cache opposite the camp office that's another good spot, and speaking of the office theres usually a body behind it that might have what you're looking for.

Just remember the spawns are random so don't get discouraged if your first few spots come up empty. I've had games where i had to venture into coastal highway or pleasant valley to find one, but mystery lake usually comes through if you check all these spots carefully. Maybe interesting - those crows i mentioned earlier are actually pretty reliable markers for finding good loot near bodies so always worth investigating when you hear them squawking overhead.