NCOWS (The National Congress of Old West Shootists) is a smaller organization centered in the United States which concentrates significantly more on historical accuracy - and that includes allowing double-action, and even semi-automatic, firearm designs which predate 1900 (in appropriate categories) but disallowing other "non-historic" designs approved by SASS ...
Rather than go into any greater detail, here is a link to the NCOWS website, where you can read up on their philosophy and rules -
http://www.ncows.org/
One thing that you should be aware of: SASS is a privately-owned "for profit" corporation, but it certainly dominates the "Cowboy Action Shooting" field ... to the point that it has even protected that phrase as a registered trademark - which is why you will see other organizations only using it with their permission, or else describing their activity as "Western Action Shooting" (as used by NCOWS, and trademarked by them in "self defence") or some other such phrase, such as "frontier action shooting" or the like.
NCOWS is a non-profit, member-run organization - i.e. truly a "club" in comparison with SASS - but generally seems to get lost in the shadow of SASS.
Another, comparatively tiny, organization I belong to is the Grand Army of the Frontier (GAF) which admirably combines my two main interests of "frontier action shooting" and Victorian-era military history/re-enacting - as summed up in the introduction to its Mission Statement:
Our Goal is to promote and enhance the portrayal of Victorian Era Military Personnel, from all Nations, while actively participating in either of the hobbies of Cowboy Action Shooting and/or reenacting, while always striving to ensure our Attire and our Personas are appropriate to this Era, whether in the 100% percent authentic arena or while portraying a Hollywood version...
In GAF I can really indulge in the "alias" aspect of things - either in my original CAS personna as "John M. 'Rattlesnake Jack' Robson" (a real-life NWMP constable of one year's service, and later a Scout with the Rocky Mountain Rangers during the 1885 North West Rebellion), or his first alter-ego Major John M. Robson, Royal Highland Regiment (The Black Watch) or in the next alter-ego I am now getting together: Cpl. John "Jack" Robson, Queen's Own Rifles of Canada (uniformed as the QOR were when they were dispatched to the West in 1885.) In case you are interested, here is a link to the GAF website (membership requires only the appropriate interest, by the way ... no fees or dues!) ...
http://www.grandarmyofthefrontier.org/
By the way, if you go to the GAF home page , you'll see an old-time photo depicting some aspect of Victorian-era military life. Whatever photo comes up is only one of a series which "rotate" randomly ... to see the others, you have to wait a couple of seconds, and then refresh the page, then do it again ... and so on. If one comes up which you've already seen that likely doesn't mean you've seen them all, just that the particular image repeated early in the random order ... I supplied all the non-US images shown there, such as this 1885 photograph showing personnel of the
Governor-General's Body Guard and the
York and Simcoe Rangers, posed with some "locals", during the North West Rebellion ...
... or this one, of members of the
Halifax Provisional Battalion of Infantry during their 1885 garrison duty at Medicine Hat ...
(Definitely part of Canada's "Wild West" history, and thus positively ideal for choosing an alias!)
(in more ways than one 
