Common sense?.... And perhaps his profession?
A is in limited supply, and it it takes a friend to tell you about it, then it really isn't all that common, now is it? And B is incorrect as firefighter is not a profession. Direct experience might be acceptable, or maybe insurance company testing.
Of the firefighters I know, they tell me that as soon as they know ammo to be in the house, they back off and save the surrounding houses.
Does anyone have any direct experience of firearms in a safe surviving a house destroying fire in the condition they were in before they went into the safe? Maybe some insurance tests, or safe maker testing that mixed the ammo and firearms in the same safe?