l. If this gun is the Long Recoil operated Black Magic it has no ports to worry about. I
I have a long action semi auto Breda 12 gauge that started doing that years ago. Only worked well with magnum loads. Then one day at the gunsmith shop I mentioned this. He pointed out one of the washers was conical in shape. This washer was just in front of the brass compression ring. When the washer is turned one way it is set for magnums but if turned the reverse way it puts less cinching on the brass compression ring and is then set for standard load. It got rid of the problem. The gun was my dads from the 50's and I'll bet that he pointed this out to me once before but I have no recollection of it.