As posted in a few threads down:
The purpose of this bulletin is to provide greater clarity on the maximum permitted capacity of cartridge magazines designed or manufactured for use in more than one kind of firearm. Note that the maximum permitted capacity of a magazine is determined by the physical characteristics of the firearm it is designed or manufactured for and the type of ammunition for which it is designed. The maximum permitted capacity of the magazine does not depend on the classification of the firearm, nor does the magazine capacity influence the classification of the firearm.
Just as the use of a Beowulf mag capable of holding 13-14 .223 rounds in a AR type rifle, its what the magazine is "designed " for , not what you can stuff into it. I wish all shot guns were 3 1/2".