[mrerick]

To expand on Ed's post, there are specific models of the Luger (Artillery, Navy, Carbine) that go with specific stock attachments. The correct combination of a model of Luger and it's appropriate stock (original or reproduction) has been approved by the ATF as not being a "Short Barreled Rifle" which would be a NFA item.

If you have an incorrect combination (for example, a 4 inch Luger and any of the stocks) attaching them creates a NFA item that requires a special transfer stamp and Federal registration.

If the ATF is dealing with you in an aggressive manner, they can claim ownership of the wrong model Luger and shoulder stock is "Constructive Intent" which can be prosecuted as a violation of the Federal NFA laws and some state laws.

The purchase or sale of an incorrect combination could also be a violation.

So.... tread carefully, especially when you post things on the Internet. Never put together the wrong model Luger and shoulder stock in public.

If your commercial Luger is, in fact, a Luger Carbine you are fine - and have a rare and valuable firearm.