Here are photo's of some testing I did to answer the same question in my mind.
Rounds were placed inside a loose 1" aluminum thin-wall tube for guidance, and set off into galvanized furnace duct sheet-metal.
Test set-up:
Impact, side one: The aluminum Blaser .357 case head completely penetrated the sheet-metal. The RF cases and some of the primers nearly did too.
Impact, side two:
Recovered rounds:
The .223 round blew out the primer and all the powder burned out through the flash hole without the case exploding, or dislodging the bullet.
Note: The thin-wall aluminum guidance tube was not even dented.
Ammo loaded in magazines would almost certainly be fully contained inside the mags.
In no case did any of the bullets have enough velocity or energy to hurt you.
The other components are a whole different story!
Primers & rim-fire cases are most dangerous, followed by pistol round case fragments.
I think you would be in much more danger from exploding aerosol cans, paint thinner, lawn mower gas, and gas-grill propane tanks in the garage.
Only a round actually chambered in a firearm would have enough velocity to hurt you when wearing FD bunker gear.
It will have the full power of the firearm, just as if it had been fired normally. That's why it's not wise to store loaded firearm's leaned in a corner, or a drawer pointed at chest level!!
IMHO: All ammo should be stored in GI steel ammo cans.
They will 100% contain ammo fragments in a house fire, while safely releasing the pressure.
That's what they are designed to do!
rcmodel