1. .22 LR-only because it includes rifles and handguns.
2. .30-06
3. .45 ACP
I hate posting anything online that says to the world that I have a firearm, let along multiple firearms. But what the hey, in for a penny in for a pound.
I think .38/.357 is #1 by a slim margin. All handguns
.22 LR in 2nd. More rifles than pistols
.45 ACP solidly in 3rd. All handguns
.44 Mag/ Spl. 4th. All handguns, but one.
I think .30'06 and .223 are tied for 5th.
Since one of my collecting 'niches' is US Military rifles , and with most makes and variations of the 1903/A1/A3 , 1917 , and M-1 Garand , the 30-06 wins with almost 2 dozen. Plus a lesser number of like chambered commercial sporting rifles.