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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Relationship between CBTO and freebore demension
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<blockquote data-quote="Boisedoc" data-source="post: 2605953" data-attributes="member: 99050"><p>Thank you everyone. This has been an interesting learning experience. There are a lot of factors in play. There is an article in which a ballistician builds a 300 WSM chamber with a .250 throat and a factory .188 SAAMI spec chamber. He loads a 180 ABLR at three different lengths; the last at 3" COAL. He then uses a transducer to measure pressure and he also measured velocity. The long throat chamber sees very little difference in the 3 loads and the SAAMI chamber sees significant. The author concludes from this that the SAAMI chamber is superior. This may be true, but I'm guessing there are more factors at play: (1) gas leak in the longer freebore probably being one. As the PV=nRT equation models what should be an inverse relationship. I did a little math on lengthening the throat using a 212 eldx from .188 to .215 freebore in the 300WSM and get roughly a .3-.45 grain increase in volume. Can figure this out because the bearing surface is still in the neck. I don't know if this is negligible or not. My guess is I am playing around with something that probably doesn't have a ton of real world significance. But it is fun to think about. I do like the idea of throating for the shortest bearing bullet and running with that. The 212 eldx actually has a shorter bearing surface than the 200 I understand. The hammer 181 has a really short bearing surface according to the drawings I've seen. It's almost equivalent to the neck of the 300 WSM so the SAAMI spec is probably ideal for that bullet. (I want to say the hammer is .334 and the neck is .313).</p></blockquote><p></p>
[QUOTE="Boisedoc, post: 2605953, member: 99050"] Thank you everyone. This has been an interesting learning experience. There are a lot of factors in play. There is an article in which a ballistician builds a 300 WSM chamber with a .250 throat and a factory .188 SAAMI spec chamber. He loads a 180 ABLR at three different lengths; the last at 3” COAL. He then uses a transducer to measure pressure and he also measured velocity. The long throat chamber sees very little difference in the 3 loads and the SAAMI chamber sees significant. The author concludes from this that the SAAMI chamber is superior. This may be true, but I’m guessing there are more factors at play: (1) gas leak in the longer freebore probably being one. As the PV=nRT equation models what should be an inverse relationship. I did a little math on lengthening the throat using a 212 eldx from .188 to .215 freebore in the 300WSM and get roughly a .3-.45 grain increase in volume. Can figure this out because the bearing surface is still in the neck. I don’t know if this is negligible or not. My guess is I am playing around with something that probably doesn’t have a ton of real world significance. But it is fun to think about. I do like the idea of throating for the shortest bearing bullet and running with that. The 212 eldx actually has a shorter bearing surface than the 200 I understand. The hammer 181 has a really short bearing surface according to the drawings I’ve seen. It’s almost equivalent to the neck of the 300 WSM so the SAAMI spec is probably ideal for that bullet. (I want to say the hammer is .334 and the neck is .313). [/QUOTE]
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Relationship between CBTO and freebore demension
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