It’s bad, don’t get me wrong, but most gun owners keep at least some ammunition on hand and gun nuts frequently stock up when things go on sale (or are just always buying cuz…. Zombie apocalypse….)
A store clerk isn’t going to be able to tell the difference from all of 5 minutes or less of interaction. Drunk sure, but even then, if the drunk has or can get the firearm anyhow…. It’s almost certain there’s at least a full mag.