“With no experience in guns”
And his reference for this is ...
...Thin air.
Per the link below, "Paddock, an avid gun owner, was known to come into the firearms store on a regular basis — typically to have his rifles cleaned..."
IOW he was firing his rifles somewhere, and likely became proficient with them.
http://nypost.com/2017/10/04/vegas-shooter-seemed-upbeat-while-buying-guns-last-week/