Skeety,
You just need to use a little common sense on this one. As long as you own an M16 you are good to go your question. When you replace any of the AR parts with M16 parts you, even though an innocent part, you begin to cross a slippery slope called "Constructive Pocession". That means that you have pocession and may have intent to construct an unregistered weapon. As a suggestion I would never store those parts in the same location as your ARs. It avoids the issue altogether.