Gun room build

Alright OB, I need some expertise from the resident gun nuts. Helping some buddies build a camp and they decided to add a gun room. Walls are full filled CMU, close enough to exterior to get a drain to plumb into the sewer system for a dehumidifier so I think I have most of that covered.

I think I am going to strip the walls with 2x4 on flat and then sheath them with 5/8 ply and the cover them with the gray carpet to look like a large safe. What other items (racks, etc) are good to add into this? What ventilation do you bring in from outside?