5.56X45mm

A little green man came down from outer space with a briefcase.
He ran into a southern boy, and offered him $10,000 for all his cows.
The southern boy agreed.
The little green man pulled a tiny little cow out of his briefcase, and the
tiny little cow proceeded to eat all the other cows.

Then the little green man offered the southern boy $10,000 for all his
horses.
After payment was made, the little green man pulled a tiny little horse
out of the briefcase, and the tiny little horse proceeded to eat all the other
horses.

After a moment, the southern boy asked "You don't happen to have a tiny
little Mexican in there, do you?"