.

I thought the thompson contender made this a mute point as long as the gun has a 16" barrel when the stock is attached.

Thompson Center vs US (I think it was a Supreme Court case)  rules that if it comes from the factory (on the books as a pistol) with a <16" barrel, a >16" barrel and a rifle stock it is legal so long as the rifle stock and <16" barrel are never on the firearm at the same time. In other words, you must replace the barrel with the >16" barrel and then attach the stock, or remove the stock and then attach the <16" barrel.

The presumption is that if you have parts combinations that make use of all parts in legal fashions, its kosher, but if parts are included that can only be combined to make an illegal object, then its not kosher.

Kharn

jmarkma and Kharn are right. The TC Contendor case went to the Supreme Court and it was ruled in favor of TC.  As long as the receiver starts out as a pistol, it can be switched to a rifle, then back to a pistol.  Just make sure to put the longer barrel on before the stock.

You can do a internet search on TC Contender and find the case.