I do not own a upper at this time but
I am thinking about it.
The 6.8SPC is a 270 caliber round. Its basically fly's with the same as a 77 SMK in .223rem with twice the hitting force.
So you are looking at 20.5 MOA to 600yds. I think the 600yd mark is the cut off for it.
A company down the road from me http://www.ko-tonics.com/index.html
only deals in 6.8SPC. I like the round and the uppers are just as well built and any I have seen. This is a great source of information on the 6.8SPC. Article in .pdf
the 6.8SPC has the same recoil as a .223 so not loss there like in most AR-10 weapon systems.
I see the 6.8SPC filling three roles.
1. home protection:
the round is much harder hitting and can be handloaded or can be found in factory loadings with some great hunting bullets. This would be a better choice over the 45grn or 55grn .223rem rounds for in a building shooting.
You can now take out your favorite AR-15 and hunt deer. the .270 cal bullets are very well known to take down deer and other wildlife.
3. Competition shooting.
As we all know a bigger hole lets you have a better score if the bullet is on the scoring ring. Most three gun matches round up. So if a .224 hole and a .277 hole both hit the same place the .277 can bleed over to the scoring line.
same argument over using a .45 ACP than a 9mm in IDPA.
hope that helps.