Cloves - as in spices. Sprinkle on floor and shelves,close door for a week, vacuum up and see where you are at.
2nd option - set a couple of open boxes of baking soda in case,close door, repeat as necessary.
3rd and best option - find the source of smell and eliminate it.
Baking soda is very good to freshen things up. Also a few apples tossed in will absorb the smell. But I would also consider a large trap. It could be Obama or one of his gun lords sneaking in to take your firearms at night leaving the foul smell.
You may have water, from hunting in the rain, trapped in the stock of a shotgun. It runs in through the action spring housing and gets trapped. Not only will it rust your gun but can mildew and stink. Just a thought. I had to drill drain holes in the recoil pads on my waterfowl guns.
Moisture is probably the culprit. I had a foul odor in my safe a few years ago but it was totally different. I had my safe open for a few hours while I was doing some work and just closed it up afterwards. A day and a half later, my wife was asking where one of her cats was, and if I had seen it. Turns out this cat had climbed into the bottom cubbyhole and I locked her in the safe. Luckily, she didn't relieve herself in there, but she did claw the carpet on the door up pretty good.