From Miriam Webster.
Definition of PROXIMITY
1
: situated close to : proximate
2
: next to or nearest the point of attachment or origin, a central point, or the point of view; especially : located toward the center of the body — compare distal
Several other websites say "next to." The phrasing seems to be a subjective measurement. There's no definition that exacts a measurement to proximity. Cabelas is down the street from my house but not next to it. Some would say that's in the proximity. I don't consider it next to my home. My garage, where I keep my safe is not next to my car or truck I park at the curb. I would consider a butt stock sitting on my work bench next to a short barrel upper in proximity. Not an upper in my garage proximal to an upper in the trunk of my car parked on the curb outside my home. One could also say a friends home or work could be in the proximity within the same city, but by definition, they aren't. Point is that it's ambiguous as to what distance proximity is measured. The point is, you shouldn't have those parts "next to" each other.
Definition of next to, which equals proximity is
1: immediately following or adjacent to
I'm just having fun with this. It makes you think. The link at ATF.gov is poorly written and not definitive and open to conjecture. The main point is, from reading it, is don't have parts next each other in the process of making an NFA item without a stamp. Having a short upper in my car is not next to the guns in my safe.
If there are presidents on this matter defining proximity, then I'd like to see them.
Lastly, I'm just having fun with this because I have nothing better to do. I'm not trying to be argumentative, just debating the verbiage here and how obscure it is. People will do what they want and figure things out for themselves. The just of it is, don't try to assemble an illegal weapon. I have several items in my possession that could be used to rob a convince store, but having those things dosent make it a process or premeditation.