No there isn't, at least not legally.
The "war on drugs" took care of that.
If it's in your hands it's yours.
Otherwise they could not have arrested a large number of mules for smuggling drugs some one else packed.
The drug example is true, if we were talking about the transport, and possession during, of guns.
But we're talking about simply holding one at a range or gun store, as in Alf's post.
Are you saying that if I walk into Wal-mart to shop, and stop and pick up a hammer off a rack, at that point is "becomes" mine?
Same with handling guns at the counter of a gun store..... Just because I'm holding it (literally possessing, true) does it magically become mine before money changes hand?
I know we're splittin' hairs here, by debating what-if's and what-might's .........
Like I said, I'm not in disagreement that it COULD happen.... just saying that RIGHT now there is a line marking a difference (because obviously you are not breaking the law by holding or shooting someone else's gun at the range).... but it would take a lot of absurd smudging of the line to get to the point we are talking about and that I hope cooler heads prevail (if there are any left).