Wouldn't the holes in the ammo can needed for the grill's airflow make it hard to contain the ashes afterwards? I suppose you could plug them up or something.
I use an ammo can to transport my charcoal. It holds just the right amount for my charcoal chimney and when I am finished, I pour the ashes back inside, add a little water and seal it up. I have a different charcoal chimney now, but here is my set-up:
Snow Peak Pack & Carry Fire Place
Sandbar Flatfire Charcoal Chimney
Steve in L.A.