I'll not dispute that! And for sure I think it is something all responsible shooters must be aware of - and the folks here I'd say certainly are. I say that because we are pretty much experienced shooters to varying degrees and mostly too carry people.
It is something I drum into newer shooters that's for sure, and one (of many) reasons why I extol full time carry - the carry piece is therefore under one's own control at all times.
The only reason it is not down as one of the above four rules (per Cooper) is - it is not per se a rule of gun handling - more a rule of gun ownership responsibility.
No less worthwhile however for having a reminder
