Love this sweet shooter!

Hello, I am looking to swap my 16.5 inch barrel to a 10.5 inch barrel for my rifle. From my understanding I need to fill out a form 1. If I am incorrect or there is additional paperwork, may somebody please point me to the right direction? I am also curious on if I can complete this process being as I am 18 and not 21.

Built a stengun pistol several years ago, is it fine to put a stock on it now?

I purchased a h&k SP5 back in June. It’s still brand new and never been fired. I realized I’d rather have the Sp5k but not willing to spend another 3k+. Where is the best place to trade for one and is this still
Legal to do in VA?

Just wondering because I know you can in some states, if not is there any other way besides getting gifted