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I stand corrected. Chrony today averaged 1138fps. Yeah it was cold, but still a little fast. 60% of rounds were supersonic, if just barely. That's out of a 18" barrel. Hodgdon says it should be 1073fps
Besides the usual 1680 Ive found that H110 of all things works really well for 208-225 grain subsonics in the 9.5 grain range. IMR 4198 works great too.
I tried several powders for my 300 BLK with 220 grain bullets including IMR 4227. All gave satisfactory accuracy but only 1680 cycled reliably with subsonic loads in my rifle.
Subsonic, but at what temperature?
It makes a diffrence.
from one of my mentors posts on it. (RIP)
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Subsonic is a little stickier proposition. At least in my experience. For starters what is REALLY the sonic threshold? Sure, there are some "standard" answers. But reality is that in practical application they are most often wrong. At least for me, it is pretty depressing to load a batch of subs and go shoot them only to hear that unmistakable "crack" as they go supersonic over the chronograph which says they SHOULD BE subsonic. And pretty frustrating to think that I have learned from that, go make a batch with a little less powder and find it will not fully cycle the bolt.
Trust me, thats frustrating.
Soooooooooooo, this experience sent me on a quest to learn what was going on with that speed of sound thing. To that end I learned something . .
1) Altitude really doesn't have any influence with the speed of sound. Temperature does.
2) There is a really handy online calculator that does all the math for you!!!
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