I wonder if you load it like a traditional mag or if the base and spring can be removed and then reinstalled. I noticed that the base looks more like a cap that can be removed. To say it would be an effort to load that mag from the top down would be an understatement! If the bottom of the mag doesn't attach to the front of the gun I'd say it doesn't meet ATF's definition of a forward grip; however, I'd get their opinion in writing to be safe from prosecution. But, I bet if one did try using it as a grip you'd end up bending the mag to the point it wouldn't feed reliably. That's a problem that happens with stick mags on machine pistols. That mag is kind of in the same neighborhood as John Dillinger's custom made full-auto 1911. Probably not as handy as it looks.