You're right about the original SPC chambers not allowing for the use of high performance ammo. To the best of my knowledge it's been many years since anyone has produced barrels using the old SPC chamber specs.
Remington did a very poor job designing the chamber because they were in a rush to get what they saw as a gold mine into the hands of the military. They were so driven by their desire to pump up their bottom line that upper management forced short cuts to be taken which neutered the potential of the cartridge. The chamber designs were unfinished and the barrel rifling was the same as the 6.8's parent round the .270 Win. All of this resulted in lackluster ballistic performance which is why the military didn't adopt the cartridge.
Remington abandoned their development efforts and these days the only ammo manufactured using the anemic ballistics of the original SAAMI approved 6.8 specs is that produced by Remington.
All of the design mistakes made by Remington have been corrected. The result is the SPC II which is a huge leap forward from the original design. In fact, I'm unaware of any manufacturers building anything using the old SPC designed chamber today.
Reloading manuals still show loads based on the SPC SAAMI specs. Folks that are into the 6.8 use the max loads in the those reloading manuals as their starting loads and work up from there. I imagine that at some point SAMMI will approve standardized SPC II specs. Obviously, you can't be a moron when working up handloads so be sure to read the manual to see if it specifies the loads as being SPC or SPC II.
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