I am in the same boat starting to build out my gun room. I am planning on building a slat panel wall on a couple walls. There are several styles of slat panel hangers for everything from handguns to backpacks and more. I will hopefully touch base here when its finished.
I stripped cement backerboard into 1x2x8 strips as the mounts. (You can use 1x2x8 boards.) I cut them down and screwed into the brick as the mount running vertically. Then, I used cement backer board, painted it a dull grey for aesthetics, mounted them on the 1x2 strips. Then, I used 2x8x16 boards to build a few shelves on one side. Don't discount shelves to hold ammo, bullets, powder, miscellaneous items, etc. It lays out nice and neat on the 4 shelves, these shelves don't have to be that far apart. (I started to uses old cabinets, but thought I could get more in the room by just using shelving.) On the other two sides I used garage hooks arranged like I wanted them. Also, leave room to store things underneath, cases of shotgun shells, extra fireproof safes for documents, gun cases, etc.
The bottom shelf on the long side, I cut holes in the back of the 2x8 to put gun barrels through, on the front of the shelf, I used a large washer as a mold and cut out moon shapes to rest guns. In the holes and half moons, I glued felt. On the bottom of the gun room I have porcelain tile from old projects. I just placed them in there, I didn't go through the trouble of grouting, maybe one day. I have a bunch of guns mounted this way on the floor. The guns I wanted to hunt with the most go on the front using the half moons and on the walls. I also hung my crossbow using the garage hooks.
By the way, if you ever let your wife store something in there, (if it's a room) just one document, one piece of nice jewelry, you will wish you built that room three sizes bigger.
One more feature to the room, I also cut a small circular hole and ran a piece of water hose through it, then caulked it off. The water hose goes to the dehumidifier I have in the room. I'm in the South, we can't be too careful with humidity down here.