The guns allowed in CAS, which is governed by SASS (Wild Bunch) are listed on the SASS web page.
The Mare's Leg is prohibited because it is NOT a rifle, and it is NOT a pistol.
Large loop rifle, yep, no advantage to having a large loop; it makes cycling the lever gun a PITA and slow.
Buntline?! How the hell would you quick draw that from your holster?! All pistols are holstered with the hammer down on an empty chamber.