No clue what a black hole barrel is capable of...
But with a few guns we did all on our own years ago, probably close to 18 years ago, Krieger tubes chambered and we assembled, I managed a 5 shot group around 1.25 inches also. At 600 yards.
What I expect and is my goal is .5 MOA for 10 shot groups, and I generally test for that at 300 yards trying to get to 1.5 inch groups. I will settle for as bad as 2, but if worse than that I pull the barrel after trying other thigns and replace it.
Fairly standard load to test barrel ability is LC or WW brass, 25.0 Varget, CCI BR4 and 69 sierra match king at 2.250 OAL.
you worry too much, the 223 is not capable of ever needing that much accuracy. if I plan on shooting beyond 300 yards even at a coyote I am using something more powerful, a 1 moa ar 15 will put your shots within 1.5" of aim at 300 yards. I like accurate guns as much as the next guy. I don't see the point of 10 shot groups either. If its not dead, killed, knocked over, shot up, or otherwise taken care of be it animal or target after 3 shots, there is something wrong.
You are a better shot than I. 1 moa is 3 inches at 300. Then add the shooter error, IE wobble, no one holds perfectly still. Thta often doubles the group size. Then add it mis judging distance and worse yet wind.... you could be off way more than 1.5 inches.
But being that I"ve shot yotes the other side of 500 with the 223 and a few other AR15 based calibers, evidently it can be done.
10 shot groups are what we shot in rapid fire competition. You test for what you shoot, at the distance you shoot. Though I admit I only shot 10 shot groups in the end testing at 600 while we shot 20 shot groups almost exclusively. Figured 10 told me enough.
So you'll see not everyone just hunts with an AR. Or just plinks. Or just target shoots.
Not that any of it matters, if whatever you do keeps you happy thats all that matters.
And if you need or want larger rounds for whatever thats good too. I have from .224 through .338 at the moment in the safe plus 2 50 cal not counting black powder or pistol rounds...
