Fortunately for you, many of us LEOs do know the law.
From the CA DOJ website (http://caag.state.ca.us/firearms/travel.php) on transporting long guns:
SHOTGUNS AND RIFLES
Nonconcealable firearms (rifles and shotguns) are not generally covered within the provisions of California Penal Code section 12025 and therefore are not required to be transported in a locked container. However, as with any firearm, nonconcealable firearms must be unloaded while they are being transported. A rifle or shotgun that is defined as an assault weapon pursuant to Penal Code 12276 or 12276.1 must be transported in accordance with Penal Code section 12026.1.
The controlling case law on what "loaded" means:
In "People v. Clark" (1996), the California Court of Appeal clarified that in order to be "loaded" a firearm must have ammunition "placed into a position from which it can be fired." It even went so far as to point out as an example of what is not loaded to include shells attached to a shotgun inside a buttstock shell carrier. ** There is a common misconception that merely possessing both a firearm and ammunition in close proximity legally equates to loaded.
ETA: If you are going to Vegas, most of the casinos are very sensitive about CCW, HR218 notwithstanding. Check with their security folks before you carry.