IME, when purchasing a gun, myself, or watching a gun purchase, a 50 round box of ammo is either purchased, or thrown in free. Yes, there are exceptions,
bearing in mind that many folks, like myself, purchase ammo in bulk, by the 1000 round case, and don't buy that 50 round box with the gun, at time of purchase, because they are going to get, or already have, 20X as much, or more.
IMO, there is something seriously wrong with the original math, here. Maybe a decimal point, 200 rounds per gun I would consider believable. 80 Billion rounds, maybe. I'd guess it's over a trillion.