If Kyle had the rifle at the riot, then clearly Black was not in possession of it at Black’s house, no? I wouldn’t be surprised if the feds argue that Kyle’s possession of the rifle was a “substantial transfer” regardless of whether the loan of the gun was meant to be temporary. As marktwain points out, the feds can just make stuff up when they want to persecute someone.
Since Kyle was found innocent of all charges, it could very well affect what's going to happen with Black....