What happens when you put a 20 gauge shotgun shell in a 12 gauge shotgun?
The gauge of a shotgun is the way the size of the bore is
measured. In this case, gauge is the number of balls of lead of
that diameter that would equal one pound of lead. So, a 12 gauge
would take a ball that weighs 1/12th of a pound and a 20 gauge
would take a ball that weighs 1/20th of a pound. In other words, a
12 gauge is a larger shell than a 20 gauge.