I know this has been asked in the past, but no numbers were ever collated.
Why the 6.8 SPC? Two main reasons - it performs close to the .308 out to 400 yards, and does this in a smaller bullet (.277 aka 270 caliber) and a smaller cartridge that fits in an AR magazine. It fills that gap of needing .308 performance at "normal" ranges but in a smaller, lighter package.
The cartridge parent case is the (obsolete) .30 Remington that has been shortened to fit inside an AR15 magazine. New and once-fired brass is available for those like me who reload... making this a more economically lucrative prospect. Reloading supplies are easily obtained for this round... standard .270 bullets are used. Barrels are readily available from the major sources.
The limitations to this cartridge are the size of the base, which is approximately .418" diameter. That's based off of the 6.8 cases I have in my brass collection, not off of a website. The only bolt that is close enough for this to work as a conversion would be the .223 bolt face. The 7.62x39 and .308 bolt faces are simply too large. I do not know what are the chamber pressures for the 5.45x39, so I am hesitant to list that as an available platform for conversion. I do not know if the metal is of a different grade than the .223 AK bolts, however visually they are nearly identical in all dimensions. I for one would stay focused on the .223 platform since that is readily available in today's market. If I can find some chamber pressure info for the 5.45, this may make it onto the list. The .223 bolt face could be opened up larger on a lathe as well as the extractor and you would have your 6.8 bolt!
Since this cartridge was tailored to fit inside an AR magazine, it goes without saying that a .223 magazine would be the ideal base magazine. As some of you may already know, I have been converted an AR follower to work in a .223 AK magazine, so I know this magazine conversion is doable, even if special followers are required.
Most importantly, the chamber pressures as listed on the Hodgdon Powder website place this right at the 50,000 PSI mark, making this an acceptable cartridge for use in a .223 action such as a Saiga .223 or Galil.
The default bullet sizes seem to be standardized on 115 grain, but different grain loads are available.
Generally speaking, the "smaller, shorter magnum" concept that spawned this cartridge seems to be a natural fit for conversion to an AK platform.
A few references:
The 6.8 SPC Directory - 6.8mm Remington SPC Ammo, Bullets, Rifles
Cartridge Loads - Hodgdon Reloading Data Center - data.hodgdon.com
*dead link, archive here: Internet Archive Wayback Machine
Now, do I use my Galil parts kit to make a 6.8, or... ?