Same as drilling the third hole or any other noncompliance with a non-trivial federal law. If you commercially facilitate it the feds will screw you as hard as they can politically justify at that particular time. If you do it as an individual then they'll probably ignore you but if you make it easy for them to bring a case they'll charge you with something that has a lot of prison time and let you settle for big fines.

Edit: Drilling the third hole = colloquial expression for altering a firearm in a manner that violates federal law.