YIKES! This has perilous potential. As was mentioned by Scorch, the 3 1/2-in 12-ga is a fairly recent load. It should be noted that the 3 1/2-inch 12-ga, being new, has a higher SAAMI maximum pressure. Using one in an old 3-inch gun is not recommended. Anything capable of shooting 3 1/2-in 12-ga maxi mags will have the shell size clearly marked on the gun.
Jim, ol' buddy, I wonder if the jam-o-matic Auto-5s were even adjusted properly (ring settings) for target loads.