Tim;
I have shot some quanity of the 12ga 2 3/4" magnums containing 1½oz of shot from guns of around 7½ lbs with no problem for the amount of shooting one is apt to do with a load of this type. In an over 8lb gun I would think you not have a great problem unless going for highest possible velocities. The "Equal Volume" method of loading black should give about 3 3/4 drams for a 1½oz setting, but could probably go up to 4-4¼ drams if desired without undue discomfort.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra