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I have two brands of factory ammo in three bullet weights I shoot regularly. So as to not generate false expectations or a bad impression, I'll just call them brand A and brand B.

So brand A is the higher velocity batch and I have it in 69 and 77 gr Sierra MK BTHP
I have 55 and 69 gr in the lower velocity batch. The 55 gr is Sierra BlitzKing and the 69 gr is Sierra MK BTHP.

In my 16"+ barrels, they all shoot accurately, with the distinctive edge going to the slower brand B. In my 12.5" SBR, it's about the same. In my 10.5" barrel, things are not anywhere near even. It by far prefers the lower velocity rounds of brand B.

At 50 yards the 77 gr shoots about a 1.5" group - that's awful. If that were 100 yards, I could live with that, but it's the brand A 69 gr that is the exception. It's inaccurate, it fails to feed, fails to extract, and leaves brass shavings (or worse) in the splines of the barrel extension. Remember all these rounds shoot well in longer barrels, even if the barrel is just 2 inches longer.

So, I claimed the 10.5" is very accurate with the right ammo and here's the pics.



There's a lot of sight in hits on this target and I changed scopes and had to be innovative to find new points to shoot at. But let me explain what you're seeing. The zero confirmation shots are the small red dots in the corners of the target. This was shot at 50 yards because I wanted to use my Steiner T5xi 1-5x scope so I kept the range short. The other hits are zeroing, adjusting, etc. and even some ammo that didn't shoot well.

Only three shots were made on each dot and some, I don't remember which is which, is with 55 gr and 69 gr of brand B. The upper right dot is interesting. Remember I fired only 3 shots at each dot yet there are clearly 4 hits on/near this dot. Well, you remember I said I changed scopes? Well that hit at 3:00 was the first shot from the scope, aimed at the big center dot!

If you look closely at 12;00 on the big black circle, there's another tight group of 3.



I think this is enough to prove the barrel is accurate with the right ammo. But, it is quite sensitive to ammo, and it appears to be a velocity thing. All the higher velocity rounds I shot had larger groups. Probably the 55 gr was the most accurate, consistent, and shot the tightest groups.

There's a phenomenon in engineering called resonance. Resonance typically occurs when everything is tuned just right to cause a regenerative effect. Resonance can be good or bad. When it's bad, it causes highly undesirable actions in mechanical systems. If you've ever run over a "washboard" in a gravel road with your car, you got a taste of bad resonance. Those washboard bumps are formed by vehicles. It turns out that the bumps are spaced at just the wrong intervals and when you run over them at just the right speed, it may feel like it's jarring your car apart - that's due to resonance. If you go slower or faster, you'll likely find the bumps to smooth out. Anyway, that's what I think is going on with the 10.5" barrel and the problematic 69 gr round - every thing is just right to produce a resonance effect.

I think a Seekin's Precision 10.5" barrel will be in today, but it will be Wednesday before I get to shoot it. It won't get here until about 6:00 pm. This is a much heavier barrel than the AR Stoner and hence the resonance will be different. It will be interesting to see how it handles the 69 gr bullets!
 
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