I've had the opportunity to test the 6.8 SPC cartridge in several AR-type rifles. And generally they were pretty accurate and potentially very accurate. The 6.8 SPC ARs that I have shot are the Wilson Combat Recon in 6.8 SPC and a Barrett REC7.
The Wilson Combat was tested while hunting and on a range, and it proved to be incredibly accurate and ultimately reliable. The Barrett was actually fielded for quite some time where possible and has been used at every department qualification for the last year. This rifle was a bit pickier about what ammunition was most accurate but with the correct factory ammunition it was very accurate, producing groups well under an inch at 100 yards.
The last platform used in the semi-auto AR test was a Primary Weapons Systems upper that was just built for an upcoming class. This was used for comparison testing on the chronograph, as it has a 12.5-inch barrel.
As noted, the bolt gun is a custom job. It was built for me by Mike's Gun Sales and Service of Aransas Pass, Texas, on a Remington 700 SA and bedded into a McMillan stock. The barrel is a 20-inch stainless, and it is tapered to fit a suppressor that will be attached at a later date. It uses a standard Remington trigger and an AICS magazine inserted in Mike's custom-built bottom metal.
For optics, I fitted this rifle with a Night Force NXS F1 3.5-15x mounted in the company's fantastic rings. I have used this scope on a couple of other guns and it is very consistent. The sister to this rifle built at roughly the same time has printed .75-inch groups with hand loads at 320 yards, so it is as accurate as you can get with this platform.