Back to your main line of thoughts, Mystic.
For vintage guns, I'd be happy to consider going by old school caliber, just tell them that, with all the corporate mergers and mass production, modern guns really have been reduced to a caliber per class and the differences now reside entirely in the tech of the gun. This helps make ammo for that vintage gun even more "special" when they find it.
As others have mentioned, caliber is a great thing to mention as part of the story of the game, to give them an idea of the enemy they're dealing with. Someone taken down by an 8 gage slug was certainly dealing with a more deranged, and stronger, SOB than the guy shot in the back of the head, at close range, with a .22. Me, I'd be more afraid of the .22. The SOB with a giant shotgun, he'll stick out, I'll see him coming. The man with the .22 is either some crazy spy from MI6 or mafia, cool, in control and prepared to kill as necessary, with the minimal fuss necessary.
Aside from that, caliber makes me shudder, it's not the flame-war shudder Medicineman likely had. It's the GM shudder from when I ran a little game called d20 Modern, where every gun was listed with its caliber and my players were the hoarder types who wanted to salvage every scrap of ammo they came across. Which meant remembering what each gun each mook used. Very good reason, IMO, to resort to "light pistol caliber", "heavy pistol caliber", etc, in the rules.