Don't know about the size of Read shells but don't forget that all but a few James rifles were called by the bore size, which was 3.8" also referred to as a 14 pounder. The table that appears on page 152 of the book "Field Artillery Weapons of the Civil War" indicates a Shenkl shell for a 6 pounder rifled gun, bore size 3.67", with a weight of 12 pounds and the Hotchkiss shell for the same gun weighing 14 pounds. Does not indicate Read shells were used.
Ron