A lower cost idea to secure a new gun room in your home. It will slow the bad guy down and force him to use power tools to get in your gun room.
Purchase an 8 foot wide roll of heavy galvanized fencing and secure it to the wall with heavy screws and very large washers or long metal strips. Floor, walls and ceiling get covered.
If you are really mean, circle several loops of romex in the wall where a bad guy might cut through the wall with a saw. Do it to code please, but it will surprise the heck out of the bad guy if he's using a power saw. Shock and awe!
When you get that done, and have returned from the hospital from your second set of stitches, its time to install a heavy metal door and frame. Tie into the frame the fencing materials, and you are ready to install the interior wall to make the fencing barrier very difficult to get in. Metal sheets or plywood covered with drywall or pegboard to make it look good are great.
If you have welding skills-all the better.
The ideal place for this room is in the corner of a walkout basement. Walkout basements are not normally prone to flooding. At least two sides are block and/or concrete, but I'd still cover them with the same materials just to be safe.
I like the idea of a normal door in front of the heavy door. You can even mount shelves to the heavy door and make this small area look like a closet. Bad guy opens normal door, area looks like a storage closet and moves on
That was the set up for the guy I worked for when I was in high school in the 70's.