I blogged about the decision last week here; the opinion is now available here. An excerpt:
[T]he text of the “handgun ammunition” definition provides no objective way … for a person … to determine if a particular ammunition caliber or cartridge is used more often, or used more than fifty percent of the time, or used for the most part in pistols, revolvers, or firearms with barrels of less than 16 inches, even though the same ammunition caliber or cartridge may also be used in rifles.
The law, as you may recall, regulates “ammunition principally for use in pistols, revolvers, and other firearms capable of being concealed upon the person, as defined in subdivision (a) of Section 12001, notwithstanding that the ammunition may also be used in some rifles.”