What size shell does a Stevens 520 20 GA shotgun take?
Whether it is a Savage/Stevens of any other modern shotgun, look
on the top or side of the receiver and it will give you that
information. For example: on a 12 gauge it will say 2and3/4 inch
only or 2 and 3/4 inch and 3inch magnum. The gauge is equal to the
number of corresponding lead (as in metal) balls that it takes to
equal on pound (troy)