That is exactly the kind of thing I would expect law enforcement to sit on.
A bolt gun in 308 with a four-round magazine is essentially a throwaway medium range precision shooting implement. I’d be very interested to know if he also purchased some glass to go with that.
If that firearm doesn’t turn up in his vehicles or homes there’s a good chance that if you find the rifle, you find his planned egress route.
enter, Bruce, stage left
http://www.tmz.com/2017/10/04/stephen-paddock-brother-bruce-rap-sheet-arson-threats-las-vegas/