Not prohibited.... just classified as a SBR (Short Barrel Rifle) and therefore Class III Firearm... means the purchaser has to pay a $200 SOT (tax stamp) to own it and pass a Federal Class III background check for approval.
This is no different than adding a Mako Stock to your Glock or certain other acccessories... changing the handgun to an SBR under US Federal (NFA) laws.
Mark