I cobbled together a gun rack to hold my security shotgun under my bed for less than $2.00 and thought I would share with you, since I have seen many fold looking for something to hold a shotgun under the bed. It isn't perfect, but I am happy with it. We do not have to worry about children accessing it.
I bought 2 hooks at Home Depot that are designed to screw into studs or rafters for the purpose of hanging items in a garage; bicycles and such. Those hooks come covered with some type of foam. To prevent them from going through the bed slats and penetrating the material covering the bottom of the mattress, I fastened some scrap wood to the bed slats with wood glue and drywall screws. It isn't perfect and isn't designed to allow recovery of the gun while in bed. (I have to drop to the floor and reach under the bed, which is good for three seconds.) Then again, I don't exactly live in a war zone.
Forgive the mess under the bed.