The Old Yooper’s statistics on range brass may be somewhat skewed in favor of automatic pistol cartridges because revolvers do not [involuntarily] eject spent rounds. Few cops are ‘gun guys’ but anyone who reloads saves their brass. This is easier to do if you don’t have to chase them, so a higher percentage of auto pistol cartridges are left behind. This having been said, I don’t think anyone will go far wrong stockpiling 9mm, . 40 S&W, and .45 ACP–both live rounds and spent brass. The pocket pistol cartridges are less attractive to reloaders, but I would not overlook .38 Special and .357 Magnum.
JWR Adds: And some revolver calibers that are favorites of handloaders, such as .41 Magnum, probably won’t be found at all, unless someone accidentally drops a piece! Nor can I imagine anyone that owns a .454 Casull or one of the new .500 S&W revolvers just walking away from their brass.