If he's smart enough to lay low until the spent casings settle, he'd have his pick of a few. Like the SgtMaj in We Were Soldiers Once ... if I need one, I suspect they'll be plenty laying around on the ground.
Whatever shtf gun you choose, even if you don't plan on having to move, I suggest you load your survival pack up as though you're never coming home (ammo, food, etc.), grab your gun, and go for a 25 mile hike in the woods on a really crappy weather day. If you really want realism, start early after a long night of drinking to get the illness affect. Then immediately shoot a small course of fire, with your pack on, right after that hike. If you still like your choice, then you made a good one. Uncle Sam made me do that a few times, not with a hangover but pneumonia twice and hypothermia once - very instructive.
BTW, if you're weapon of choice uses accessory rails, I recommend zip ties. Lots of acogs, laser designators, broom handles and surefires were lost all over Iraq and Afghanistan when the mount failed under normal operating conditions. And those mounts don't insure point of aim if the weapon is dropped, used as a club, or breaks your fall ... all of which are normal in combat.