4 rawl bolts should do the job fine
It's been a long time since I needed a gun safe but this is what I did.
First I had a safe built to my specifications (7-lever locks to each of the 3 compartments, of course) - especially size. It only just fitted into the back of the cloakroom to the extent that I had to remove the skirting first!
Then I bolted it to the back wall with 4 of 10mm diameter rawlbolts. For extra security I added another 4 of them through to each side walls and then several heavy duty screws through the bottom into the floor joists. Finally I cut the skirting to size and refitted it.
When we moved out it was just too much hassle to shift it, so I left it behind....
"I would laud the day when there is a rifle in every cottage in England." Robert Arthur Talbot Gascoyne-Cecil.Your 'local' hospice needs your charity . Please give until it hurts..I made this