My lord -.-
Okay, remember when I said the whole thing about driving different cars? It can be don't without taking a formula one course training on it. It simple and basic to operate another vehicle. It's the same song and dance.
Same with handguns. Same darn thing. We're gun enthusiast. Your gun most likely has the same function of another gun. In that rare rabbit out of the hat scenario, you'll more than likely be able to use that outside weapon. It'll be mediocre, and not as well as the weapon you've trained extensively with. But bullets will be flying out of that barrel nonetheless. Keeping you in the fight.
Unless by your logic once again, you stop time and say "Be right back. Let me run a magazine through this at the range."
I tend to agree with Constantine's point and will add that there are only so many ways to fight with arms, its not all that deep. Should you be reasonably proficient with your selected platform? sure.. Should you have a good understanding of the methodology of fighting tactics and a fighting mindset.. sure. Do you have to be as good with all manner of weapons and arms in order to be formidable? No, I think that is over-intellectualizing the process. I believe that there is a fighting science, I will not dispute that but when people begin to describe 2 men fighting in an alley like a Military campaign, I consider it equally over thought.