That's a fair statement, as it was just a quick calculation and I can't personally attest to accuracy of the numbers.
That said, I do believe those numbers are pretty reasonable, assuming the source data was good. Here's how I got those numbers:
I didn't have any chrony data on the 13.5 directly, despite having a very similar build a while back (was actually my first AR platform self-build, one of the few I regret having to sell; old pic at bottom of post).
However, I was able to find some data published in the following TTAG article, and used the published speed of 2380fps for the 168gr projectile:
https://www.thetruthaboutguns.com/2...t-barrel-length-muzzle-velocity-and-accuracy/
(graph from article - mods feel free to remove if necessary):
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I then just used the 2380fps MV in my BC and ran the tables with atmosphere...pretty straightforward, and a good starting point short of rifle/load specific data...
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Didn't include links here to studies on bbl length harmonics as figured would be too much for one post...
And pic of my old similar build (this one had 13.7 bbl with pinned comp = 16.25" oal):
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I’ve read the articles on harmonics and find them pretty interesting. I feel that info is accurate.
I’m curious what you used for the ballistic coefficient and whether it was G1 or G7.
Running Sierras G1 variable BC and 2400fps velocity and my current altitude with a generic atmosphere I get subsonic at 813 yards.
Using the G7 BC from StrelokPro I get closer to 750 yards.
That said, the real killer is the same thing that plagues all .308 at longer ranges. For a standard .308 running 2700-2800fps MV you have to deal with the 800 yard “wall”. When the bullet seems to be hit by a sudden gust of gravity and your elevation must be dialed in nearly perfect. Starting out 300fps slower you will hit that wall much faster. I show a 50” drop from 750-800 yards using the Sierra variable BC data.
I used “Ballistic” for the calculation because it’s easier to play in than StrelokPro. Strelok does a better job of predicting when a bullet will enter the transonic range though IMO and usually says it happens a littler earlier.
And I’m not bashing .308 in any way. I stuck with it for a long time while guys I shot with got on the Creedmoor bandwagon. Ran my 18” barreled Savage to 1k using 175gr SMKs but never got further. It seemed to be a shotgun pattern at 1,200.
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