As long as you don't conceal one on your person. If you do, it becomes an AOW. And, I think that is BATFE's reasoning for treating "firearms" differently from "pistols" that have "stocks that aren't stocks" that are actually used to fire the weapon from the shoulder. "Firearms" don't fit into any GCA or NFA definition, so when concealed on the person, they become an AOW, assuming they don't fit into one of the AOW exemptions. I assume BATFE's reasoning is that a "firearm's" nature can change based on its use in the AOW sense, so why wouldn't firing it from the shoulder also change it into an SBS? I think this is incorrect based on the differing definitions of an AOW and SBS. AOWs need only be capable of being concealed on the person, but SBS are supposed to be intended to be fired from the shoulder. I would think the prior letter holding that "misuse" of a brace by firing from the shoulder does not change the intent of the product would apply equally to the definition of an SBS.