How about the 45 ACP conversion cylinder? Is the 45ACP a cowboy load (cast lead bullets) that is allowed in cowboy action shooting matches?
Technically, yes, 45 ACP is allowed in Single Action Shooting Society sponsored Cowboy Action matches. I have pasted in the definition of revolver calibers allowed by SASS at the bottom of this post. Notice that it is not an all inclusive list, it simply cites some examples. There are plenty of other revolver calibers, such as 45 Schofield, 44 Special, and 38-40 that are legal but are not on this list. The key here is the phrase 'commonly available in revolvers'. Revolvers have been chambered for 45 ACP since 1917, it is a well recognized revolver caliber.
You may get an argument at some matches, but at least at SASS matches, 45 ACP has been ruled legal, and this has been discussed a great deal on the SASS Wire.
I doubt if it is a legal caliber for NCOWS.
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.