Why not store with bolt open, on a full mag? Serious question... (New AR15 owner here...)
there are a few AR15 safety gotchas that aren't obvious. remove the magazine from your rifle and verify the chamber is empty. lock the bolt to the rear. point the muzzle up and make sure 'up' is a safe direction. hold the rifle muzzle up about 3-6" off the ground. let go.
now, imagine if you'd had a loaded magazine inserted. normally not a huge deal, but combine it with any one of a number of other unlikely events and you could have an unfortunate ND with what you thought was a rifle in a 'safe' condition