I had a 6mmART40 Krieger upper once, it was very accurate. I ran it hot which was a mistake because doing so caused the barrel to loosen from the extension after a six months or so and the reliability wasn't the best. (I think the high pressures would make the case body grab the chamber wall) ???
Once I got that fixed and loaded to normal pressures it ran well. I won some night matches with it and finished #2 at a bigger tactical match against bolt guns. That was back when they had that stupid rule that if you ran a semi auto you had to unload between movement. If only I could have just put the safety on instead, arggg.
I lent it to friends at times. One guy kept hitting the 10" plate at 807Y over and over, while the rest of us with our bolt rifles weren't so lucky. Another friend got the coldbore and follow up shot at the beginning of the match. Maybe 10% of us got it that day, it was around the same size at 700Y.
I originally was using 105 Amax at 2875 fps, which was the hot load and very accurate, slowed it down to low 28's later on and it became reliable but not as accurate. When the barrel got worn I slowed it down even more to 2740 fps which cut it's balls off in the wind compared to the hot load.
I use a FatRat now with 95SMK's going 2870 fps. With the Wilson barrel it's not quite as accurate as the other upper but is close in the wind and recoil is noticeably less. Though part of that is it's braked despite the other upper being very heavy.
Just sharing my experience with these cartridges.
FYI, I had a 243 DPMS large frame and the heavier BCG coming back moved the rifle much more than the regular AR BCG.
