I agree with the angle grinder approach. Did that to a safe that was given to me. Sentry and other inexpensive homeowner Fire safes typically have very thin metal on the inner and outer walls with a fireproof medium sandwiched in between. Sometimes it's fireproof sheetrock (gypsum board), and sometimes it's a moisture laden material that's designed to give off wet fumes while just sitting 24/7, and definitely when heated during a fire so as to protect your objects by bathing them in moisture (don't put guns in a fire safe...they will rust). The inner and outer walls are often not much thicker than a couple of sheets of paper, believe it or not. The harbor freight grinder approach is probably your least expensive approach. If the walls are thicker than normal, you can always return the grinder, and you are only out a few bucks for the replaceable grinding wheel/disc.