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MileHighShooter
Posts: 4,997 Senior Member
Big bore twist rate
MileHighShooter
Posts: 4,997 Senior Member
Gettin' ready to order my barrel for the pre-64 project. I've found a few barrel makers that offer both 1:12 and 1:10. The 12 seems to be the world standard for the last 100 years on such matters, but that was also a LONG time before all these fancy new bullets long as a Vista Cruiser. Getting kind of hung up on which one to go with. My plan is going to be mid weight but higher BC/longer mono's and Accubonds.
What do you guys think? Not exactly a long range round, so I won't likely be dropping any of those new SMK's or Bergers in it, but the new 300gr Accubond and the 300gr TSX aren't totally out of the picture. I'd like to play with the 350gr Woodleigh bullets, but those aren't really all that long.
What do you guys think? Not exactly a long range round, so I won't likely be dropping any of those new SMK's or Bergers in it, but the new 300gr Accubond and the 300gr TSX aren't totally out of the picture. I'd like to play with the 350gr Woodleigh bullets, but those aren't really all that long.
Replies
Jerry
375 Ruger
About time!
Is most likely up to 300 gr, really heavy for caliber though? Seems like 300gr in 375 is the 180gr in 30 cal :uhm:
I know I said I'd like to toy with the 350gr Woodleigh but its a rather compact flat base bullet, shorter then several of the 300gr boat tails, so I'm not sure if that really fits in the heavy for caliber arena, due to OAL
Hush you, I was recovering from an expensive hunt lol But I got some side work this week, so figured it was a good time! And that project has been at the smith for over a month now and he has detailed written instructions as to how it'll end up.
ETA - the only thing we haven't fulled covered as of yet (giving him some time to catch up after his hunting rush) is specific brands/prices of the parts. Once I get the barrel down then I have to pick out the bottom metal....that one is giving me a headache currently.
Well that is the conundrum right there, it HAS been working for a long time...but, do you think it would run the chance I can't play with some of the new bullets coming out that are quite long? That 300gr Accubond is HUGE. AB's and TTSX's and such, weren't around even a decade ago.
A few with some of the 350gr Barnes and the NF bullets, but that has been from H&H's. Haven't found much of any real data with the faster 375's and these newer bullets
OR I could just look on their site, since I forgot they have detailed load info :bang:
Another odd thing on their page that REALLY is baffling me....compare the date for the 375 Ruger and the 378 Wby. Not the 375 which is more or less a 375 H&H AI...but the 378, the belted Rigby case. Ruger with top loads is going 2600-2700, 378 is going 2600-2800. And it is using nearly 20 grains more powder in some loads. I thought the 378 was the king of beasts? I mean 3 of the loads are over 100gr, one up to 115gr and that is 2650. Dang, talk about diminishing returns.
My guess is the 1:12 twist will do just fine for what you plan to do. I played with some numbers and came up with this:
Woodleigh's HS mono .375 cal. 300 grain bullet (1.39 inches) at 2650 fps in a 1:12 twist gives a stability factor of 2.995 or safe stability (less than 1 is unstable, 1-1.25 is a "maybe", depends on conditions, 1.5 is considered gtg for military use and 2 or above is considered safe for nearly all realistic conditions).
Next, I tried Woodleigh's 350 grain FMJ (1.447") at 2400 fps (guessed at the velocity) and I get a stability factor of 3.013. GTG!
Finally, I tried Woodleigh's VLD Chey-Tac 350 grain (2.110") at 2400 fps and got a stability factor of 1.005 which I consider "no-go" but using a 1:10 I get a stability factor of 1.447 which is close enough to be used for military work.
All calculations assume STP (59 degrees F and 29.92 in of Hg)
You can do the calculations yourself for other bullets here: (you need the bullet length)
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
You can read more about miller stability if you're interested here:
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf
http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_2.pdf
So unless you plan to use some really long bullets, my guess is that a 1 in 12 will do just fine.
Comparing the SDs is an indicator:
.375 300 grain SD=0.305
.375 350 grain SD=0.356
30-06 180 gr. SD=0.271
30-06 200 gr. SD=0.301
30-06 220 gr. SD-0.331
Just my opinion, but that seems pretty good for that caliber.
I suspect advances in controlling powder burn characteristics mean we can make more efficient use of cartridge capacity, it's all about getting the best pressure curve for the volume (efficient use of available powder--less waste).
JMHO
Hope some of this helps out.
How easy it is to make people believe a lie, and [how] hard it is to undo that work again! -- Mark Twain