Manufacturers of 6.8 SPC guns and ammo have been confused about the barrel specs because Remington had problems with the initial loads released for consideration of military adoption. They used a SAAMI drawing and barrel blueprint which showed tight chamber, along with the 1:10 inch twist, and that, exacerbated by powder that was temperature sensitive, resulted in pressure problems right from the start. Unfortunately, SAAMI seems to have followed the “Greenhill formula” recommending a faster twist than was necessary, without anticipating that this could limit the velocity potential of the cartridge
Then came the Silver State Armory X-treme cartridges. SSA was asked to make the bullet by an outside source. The bullet was copper plated lead and not jacketed, and this allowed for the swelling of the bullet upon firing, which caused obduration in the throat, and thus a small ring of copper could be shaved off into the throat. Upon firing subsequent rounds, the bullet would encounter this reduced “luminal” diameter and a pressure spike would occur, causing blown primers, and stuck casings.
Finally, some pretty ingenious folks designed a chamber that would allow for extra room, so that the broad shouldered bullet would not spike pressure. The diameter of the “SPC II” ** chamber as we will call it, is slightly larger than the SAAMI chamber. It is also a little longer. However, since so many guns had the tighter SAAMI chamber and 1 in 10” twist, SSA finally withdrew the X-treme from the market. It seemed, at that point, that the future of the cartridge was in question, but it wasn’t just because of one bullet. It had more to do with the confusing SAAMI specs, and a twist rate that was based upon a long action bolt gun cartridge, using bullets which were 40% heavier.
By 2006, manufacturers were claiming “a .270 Winchester uses 1 in 10” twist, so that’s what I’ll make mine.” That, in conjunction with the shorter SAAMI chamber, combined to give the highest possible pressure environment for even moderate loads. For this reason, shooters got factory ammo from Hornady and Remington (now just a bystander), that had to be “watered down” to assuage liability concerns…and actually, you can’t really blame them, as most of us would have done the same thing, not knowing who would shoot what, out of what gun, and with 3 or 4 different barrel designs on the market.
SAMMI spec barrels. 
