Weren't the Browning mag 20s only supposed to be fed magnum ammunition? Dad also owned a Japanese mag 12, and I seem to remember reading in the directions it was a magnum only gun, or, words to that effect. I don't remember Dad having success with light loads with the gun.
My Dad sold it, like new in the box, to a neighbor kid to fund his SBE purchase. I doubt the thing has been fired since that day.
The Browning A5 20s have become really expensive.

Best,
Ted