RE: What 3 inch 6 shot shell?
What choke/constriction is that old single?
If it has a fixed constriction, you can put your wallet down because you don't want to shoot any of the high density stuff down it.
My two favorite lead/copper plated #6 shells were always Win Supreme (black box) 2oz 3" #6 and Rem Premier HV 3" 1 3/4oz #6. Killed quite a few with both of them.