Pointing at an exterior wall is safe? Really? Who's beyond it? Do you know?
A loaded masonry exterior wall will stop a single round from any gun I'd be looking at. So will a concrete ceiling, and a gravel-and-tar-over-metal roof probably will as well. Obviously if your gun shop is a corrugated metal building or vinyl sided plywood or something, or if you are on a lightly built second floor with people beneath you, adjust your directions accordingly. But since the point of watching your muzzle direction is to minimize the risk to other humans if a mistake should be made, then yes, sweeping an exterior wall is
vastly preferable to sweeping an unshielded human being inside the store.
The whole point of the Four Rules is defense in depth; even if someone breaks one of them through carelessness, fatigue, or ignorance, *multiple* rules must be broken for a tragedy to occur. For example, if somebody gets careless with the muzzle because "it's unloaded" (Rule One violation), and they allow the muzzle to sweep an unshielded human being (Rule Two violation), Rule Three (finger off the trigger) should prevent a tragedy until range officers or bystanders can correct the Rule One and Rule Two violations.
What I'm seeing advocated in a few people's comments upthread is the
de facto abandonment of Rule One and a loosening of Rule Two, and I strongly disagree with that.
And aside from the safety ramifications, I can't reiterate enough how
rude it is when someone sweeps other people in the room, even briefly, with an "unloaded" gun. I don't care how many times you've checked the chamber, I haven't checked it myself, so I'm not going to take your word for it.