From the Shooter's Handbook:
REVOLVER CALIBERS
Must be centerfire calibers of at least .32 caliber and no larger than .45 caliber or percussion calibers of at least .36 caliber and no larger than .45 caliber.
Must be in a caliber commonly available in revolvers. Examples include, but are not limited to, .32-20, .32 Magnum, .357 Magnum, .38 Special, .44 Magnum, .44-40, and .45 Colt.
Standard velocity .22 caliber rimfire ammunition is allowed within the Buckaroo Category only.
Although the .32 caliber revolvers and .36 caliber cap and ball revolvers are legal, they may not be powerful enough to handle all reactive targets.
RIFLE CALIBERS
Must be centerfire of at least .32 caliber and not larger than .45 caliber.
Must be in a caliber commonly available in revolvers. Examples include, but are not limited to, .32-20, .32 Magnum, .357 Magnum, .38 Special, .38-40, 44-40, .44 Special, .44 Magnum, and .45 Colt. The only allowed exceptions are the .25-20 and .56-50. No rifle calibers such as .30-30 or .38-55 are allowed.
Buckaroo/Buckarette Category competitors choosing to utilize .22 caliber firearms must use standard velocity .22 caliber rimfire ammunition only.
A power factor has been in force for a few years now:
The minimum standard for center-fire ammunition used in all smokeless categories in all SASS matches State, Regional, National, International, and World Championship Competitions is not less than a minimum power factor of 60 and no velocity may be less than 400 fps. The maximum velocity standard for revolvers is 1000 fps. The maximum velocity standard for rifles is 1400 fps. Pocket pistols, derringers, and long-range rifles are exempt from the power factor and velocity requirements.
One thing that makes CAS shooters get a bit hot under the collar is folks wanting to join and change the rules right away, or change the rules without joining at all. All shooting games have rules. You can't show up at a SASS match and shoot your S&W Model 10. Don't have centerfire guns? If you want to play, you must shoot centerfire. That's in the rules, and believe me, it has been debated over and over again for years.
I would guess that a large percentage of those .38 shooters would trade their centerfires for rimfires if the change was allowed and that doesn't seem to sit well with some.
That's your opinion. I have been shooting CAS for over ten years now, and I have never heard anyone say that. And some of us elect to fire large calibers loaded to the gills with Black Powder, just because we can.