I have been searching long and hard trying to find a source for my assumption about the bore size. I found this one in
Wikipeida, but will continue to look for another. I was incorrect, according to this, in that is was weight in lead not iron.
"This leads to certain guns being referred to as 6-pounder, 25-pounder and so forth. However this relationship between calibre and projectile weight changed with the introduction of the cylindrical rifled shell. The gun continued to be named by the weight of projectile it threw although this no longer gave a direct indication of the barrel size"
Caliber - Wikipedia, the free encyclopedia