Jerry & others:
If I may I would like to add my thoughts and findings to this discussion. I have done extensive research on the 6.8 spc, and do own a Stag carbine in this caliber.
At its inseption, a grass roots movement right at field level, thos concerned were looking for looking for a more powerful cartrdige with better stoping power, ideally in the same foot print as an AR, but definately smaller than the AR-10.
The mission parameters for this carbine were to mid-range solution (about 300 yards), and to be to be "one shot" leathal in an anti-personal roll inside that range, as well as offer vastly superior penetration on hard targets when compared to the 5.56. Sever different cartridge variations were tried, and calibers includeing 6.5, .277, and 7mm were tested. In the end they setteled on the 30 rem as a parrent cartidge, which requried an "opening up" of the standard 5.56 bolt face, but still remained fully functional both in Semi auto, and full auto fire.
As Jerry indicates, they found that the 6.5 carried the best, and was most accurate out to ranges well beyond the "midrange" paramets. They also found, that based on the cartrige foot print and the bullet weights use, the 7mm had the best prenetrating power.
When comparing the raw data, what the testers found was that the .277 bullet carried "Almost as well" as the 6.5, and penetrated "almost as well" as the 7mm bullet. As such a compromise was struck for the purposes in question, and they settled on the .277 caliber. For use in AR carbines, they settled on bullet weight from 90 grains to 115 grains, with most of the FMJ ammunition I have come across being in the 115 range. At the end of all this testing what they found is that the 6.8 SPC had 80% the power of the 308 (7.62 Nato) with only 20% more recoil than the 223 (5.56 Nato). Best of all, all that was required to use this cartrige was a new upper, and slightly different manufactured magazines. This allowed easy switch outs, and allowed combatants to use the same muscle memory.
On a technical note, the 6.8 cartridge itself went through at least 2 variations, and early carbine barrel had a 1-10 twist. Pretty well all ammunition manufactures (including black hills and Remington - as far as I know) have settled on a the "spec 2" variaiton of that cartridge, and carbines are now produced witha 1-11 twist in a effort to better stabalize the bullest used and reduce preasure (there were early accounts and reports of over stabaliztion of bullets which resulted in poor and unexpected preformace). These last two changes or tweeks seem to have address the most critical issues, and as far as I know, is preforming very well in its current role with great success.
On a presonal note, I considered both the 6.5 grendel as well the 6.8 SPC for my "Very diffent" AR. At the end of the day I settled on the 6.8 because due over all cost, and ease of abbility to get components, mags and parts.
Just my 2 cents.
regards
AbH