5.
5) Brute force.
Since most gun safes use a bent body shell, the only welds that are available for attack are on the top and bottom caps. This is the most common method for gaining entry to home safes. It's loud, it's hard work, but once the safe has been knocked over onto it's face it's just a matter of a little sledge and pry bar work to pop the top off the safe. This only works because most low grade safes don't use full length welds, only a small fraction of the seam length is welded. You may be told by some safe manufacturers that their bent bodies are stronger than a welded body. The report I have gotten from people who weld is that a welded seam will be stronger than the bent corner, provided more than just spot welds are used.
Defeating this attack is mostly a matter of keeping the safe bolted down so that it can't be knocked over. Make sure the welds on the top and bottom are full length, or nearly full length welds, and not simply spot welds.
On most of the commonly available commercial safes, the body is 10 gauge steel, about 0.140" think. This thickness of metal can be attacked successfully with axes or splitting maul. Thus the thief just hack their way in with an ax. A heavier wall thickness is the only defense against this form of attack. I would guess that a 1/4" body thickness would be sufficient to defeat this form of attack.
Pry attacks are also in this category. Pry bars are inserted into the crack between the door and the safe body. Then, they attempt to bend the safe body away from the door, or bend the bolt carriers, rip out the door box, etc. Browning has very light sheet metal that holds the bolts, and when the door is forced open, the bolts rip though the door box.
A variation of that kind of attack is to use something similar to ``the jaws of life'' that are often featured in TV drama's to pry the doors off of cars. These hydraulicilly operated jaws can be inserted in the door-body crack, and the whole safe pride open with a minimum of fuss. These devices are not cheap, and it seems to be more of a theoretical concern, since none of the safe people I've talked to have heard of such an attack being attempted on a home safe in their area.
Minimizing the door-body gap, and having a good strong door box and door sill will minimize the effectiveness of this kind of attack.
6) Take the safe.
If the thief can take the safe with him, he can spend all the time he wants to get into the safe. Make sure you have the safe bolted down in such a way that it can't be removed from outside the safe. If bolting down the safe is impossible or impracticle, a large amount of weight can have the same effect 300 lbs. of lead in the bottom of the safe is going to make it much less portable.
B) General solutions
There are several things you need to do to protect your investment, other than the safe itself:
1) Get an alarm system. If the burglars hear an alarm go off, they are less likely to stick round to finish the job. Alarms on both the house and the safe area are a good idea.
2) Hide the safe. If the safe is built into a wall, or behind a false wall and they can't find it, they can't break into it. It may help to keep a cheap gun cabinet around with a couple of junkers guns in it, in order that the thief that know you have guns will find it, break in and leave disappointed, without looking for your real safe.