As a recent Supernova Tactical owner, I've been researching this topic a lot. Typical IANAL disclaimer, but while the telescoping stock is fine under AW regulations because it is a pump-action, it should also be fine under 922r, because it is "...or readily adaptable to sporting purposes."
I can change out the stock on my Snova in under 5 minutes. Pretty "readily adaptable" to me.
Given that interpretation, so is a mag extension that be screwed off and replaced with the stock cap. (Just keep the stock cap & butt handy!)
Of course, from my discussions with various shop owners and law enforcement officials - if the powers that be are in your house looking for 922r violations - you've probably already got way bigger problems than a possible 922r violation.
