If you are not opposed to do some under house work I can easily explain the process off this forum for you. You can also higher a contractor to enforce your foundations in this location for minimal cost too.
The choice is your and I will provide the upgrade information as soon we know the floor joist configuration. I would also need the length and width of the gun safe (footprint as it would sit on the floor). My safe is 2.5' by 3.25' and its a Knoxx safe. But is only weighs about 420 pounds.
My entire house, roof and second floor has 2x12 at 12 inch on center. This makes a strong attachment and 1' ACX decking for mid floor and 3/4" ACX for roof decking. A little over kill but I had it build for possible upgrades. The walls are 2x6 also for plate support (double in strategic locations) sitting on a 10" concrete slab.
In older houses they normally placed joist at 16' OC and that is not a bad thing but not able to support 1000LBS over 8 to 14 SF.
Placing casters under that heavy of a safe is also a risk as you concentrate the load on very small rollers but it can be done as the choice is yours.
One last item USNAVDOC, I retired form the Navy Seabees in 1996 with 21 years (Retired CEC I am now a Structural Engineer, I know don't ask) so I would be more than happy to help a fellow service person!
Last edited by Winterhawk56; November 8, 2009 at 03:50 PM.