We have something here called a leaf cutter bee. It will fill up holes with leaf bits and lay an egg in the leaf litter. Whether the larva lives or dies it makes a mess of metal parts. Could see one of these really doing a number on a rifle bore. If the bee can come through holes in window screens or whatever, I think they could nest in a rifle store inside or in a barn. On tractors all the tapped holes with no screws will all get filled up. We keep a compressed air nozzle by the front and back doors of the shop. Sometimes they get filled over a week end.