The bars were originally made to drop the weight specified in drop shot, not chilled or magnum. Drop shot is as close to 100% lead as is possible. Antimony is at about 5 to 6% in magnum and is lighter than lead, so the charge weight will be less. Chilled is also slightly lighter than drop shot
Also the size of the shot makes a difference as there is less air space around 9 shot than 7 1/2 or bigger.
Some of the loader companies actually make bushings for 1 1/8 ounce of 9's and a different one for 1 1/8 ounce of 7 1/2's.
Even MEC offers a Skeet bar to get exactly the weight of 9's for the competion Skeet shooters that want to get every grain of weight into there reloads.
