Yeah, this guy is going to turn himself in (not). From what I’m hearing from my co-workers, having possession of an illegal machine gun gets you 10 years in federal prison on each count, so this dude is facing a life sentence. They’re going to have to catch him, and they probably will. Between his handwritten apology letter and whatever fingerprints he left on the case, they’ll find out who this dude is. The only chance in hell this guy has of not going to the clinker for a real long time is that he gave the rifles back.
If he lawyers up quick enough, the story should be that he found the box and took it home, but was busy and didn’t open it right then. When he heard the news, he found out what he had, and gave it back as quick as he could, but was too scared to turn them in to the cops.
Of course, if it turns out its the guy driving the truck, that probably wouldn’t work.
Unless the government either can prove something about how he came to have possession, or cheats by withholding from the jury the fact that he gave them back prior to his having been detected, I can't see any basis for conviction. Had the guy been caught before the weapons were returned, it may have been hard to believe a claim that the only reason he possessed the weapons was so that he could return them. But given that the weapons were returned, such a claim would be hard not to believe. I know the government routinely cheats in voir dire, but the return of property to its rightful owner has long been recognized as a legitimate reason for possession.