That damaged safe is easy to believe when you consider that 12 gage is only 1/10 of an inch. These guys
http://www.drakesafe.com/ use 1/4" (<3 gage) for everything, and it's not a cumulative thickness, it's all in 1 piece. They use very few seams.
It looks, to me, to be a pretty rationale way to build one without going overboard.
Material thickness:
STEEL
Gage (inches) (mm)
3 .2391 6.073
4 .2242 5.695
5 .2092 5.314
6 .1943 4.935
7 .1793 4.554
8 .1644 4.176
9 .1495 3.797
10 .1345 3.416
11 .1196 3.030
12 .1046 2.657
13 .0897 2.278
14 .0747 1.897
15 .0673 1.709
16 .0598 1.519
17 .0538 1.367
18 .0478 1.214
19 .0418 1.062
20 .0359 0.912