indexing rounds is a common match procedure, all calibers, not just .50
I don’t know of any work on the wildcats, the .50 is such a small competition world compared to the NRA matches, F class, etc. There have been some writeups in Very High Power Magazine over the years about some of the variants.
Most FCSA stuff is done with the BMG. The wildcats that I recall were:
.50 DTC (current)
.50 FCSA (Skip Talbot, no longer shot, .50 BAT Improved)
.50 McMurdo (shot only by Lynn M and Scott Nye)
.50 Longson (only shot by Bruce Longson)
.50 Kyser (don’t think it was ever used in competition, more like prototype)
.50 BMG Improved (Full length BMG, blown out case like the Ackley, only shot by Eric Williams, no longer shot)
There was one additional wildcat, but I don’t recall the designation. It’s not an active competitive caliber either.
Differences were varying case lengths, driving varying case volumes, but also varying case necks, differences driven by which projectile was being loaded. (smooth bullets like Hornady AMAX requiring different case neck retention than, say, a driving band equipped bullet)