Were I you, I'd create some kind of weak wall in that freezer to allow pressure to escape, in the event of the primers being ignited.
Modern, gunpowder storage buildings and lockers are designed with a weak side. If the powder ignites, the weak side gives out and allows the pressure to escape, so building or locker doesn't explode like a bomb.
Cans of powder are also designed that way, with a weak section, to discourage pressure from building so high that an explosion results.
I would assume that ammo cans are designed with a weak section, but I've never quite understood how that can be when the lid locks down tight. And if you throw a lock on that lid, you prevent it from rising up and allowing pressure to escape.
Does anyone out there know if ammo cans are designed to allow pressure to escape, even if they're locked? I would think that the military would certainly design them this way, but I can't picture what happens to release pressure.
So, to design a weak wall in your freezer to release pressure, I'd drill a 3-inch hole on each side, near the top, and put in a plastic cap with grout. Epoxy might offer too much resistance. If pressure from igniting primers suddenly built up, such caps should pop out and relieve the pressure. At that point, you'd have a slow, not burn but not an explosion.
A good container for storing powder is a wooden box, with a side panel nailed in place. Pressure builds from the powder igniting, and the whole panel pops off because the nails don't hold it as securely as screws.
Perhaps I'm overstating the danger but the point remains: don't unwittingly create a bomb that might go off in a house fire, or by itself for whatever reason.