It wasn't a straw purchase. He paid for the gun with his own money, with the intent of basically gifting it to a local police department.
I still think it could have been a straw purchase. It's entirely possible that the purchase is being funded by some anti-gun group, or that he his receiving money for the publicity stunt. So, that would mean he is not truly "gifting" the rifle - the money is just being paid by a different party than the one he is gifting it to.