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Old December 7, 2009, 11:09 AM   #1
300magman
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.243 Twist Rate

I previously posted this in the general section, but I didn't get any replys that were on topic so I'll give this a try here.

I'm wondering, If I have a fast twist barrel (about 1 in 8") that is designed to fire heavy bullets like the 105gr nosler competition bullet, can I still fire light 55gr bullets out of that barrel (at 3800fps+) or will the rapid spinning imparted on them cause them to fly apart?
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Old December 7, 2009, 12:04 PM   #2
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possibly...
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Old December 7, 2009, 12:17 PM   #3
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best bet would be to pick a Sierra bullet from their web site and ask Sierra how it would perform in your barrel. They are usually good about this sort of thing. I suppose you could compare an equal weight bullet from a different mfg but I wouldn't put any money on it.
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Old December 7, 2009, 12:22 PM   #4
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342000 rpm is more than 2 times the recommended spin rate of 55gr jacketed bullet. I am pretty sure it would self destruct (363600 G's)

Last edited by Crapulence; December 7, 2009 at 04:15 PM.
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Old December 7, 2009, 12:36 PM   #5
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Too fast with a light bullet can spoil accuracy by causing core stripping, which is where the jacket is spun faster than the core in the bore. A bonded core bullet should withstand it better than a standard cup and core bullet. It does deteriorate accuracy. Harold Vaughn was able to use a magnetometer to measure it in bullets that were running around 3200 fps, IIRC? I think they were maybe 6 mm, too, but I'd have to double-check. May have been .270's.

In addition, when you spin a bullet too fast, every tiny imperfection in mass symmetry is exaggerated as wobble. It is the trade-off between that and fast settling that caused Vaughn to estimate a gyroscopic stability factor of 1.4 was about nominal, while Don Miller put it about 1.5. Close enough to each other that I usually put 1.45 into my calculator for nominal results.

Your 8" twist at 3800 fps with the 55 grain Sierra BlitzKing, for which I show a length of .747", would give a stability factor of 3.66. Sierra recommends not exceeding 3.00, generally, so I think your in potential problem territory. A 12.7" twist would give 1.45 with that bullet at that velocity.

I don't know if the bullets will actually fly apart or not? If they "disappear" on their way to the paper, then that's what happened. You'd have to try it to see? I would not expect great accuracy, even if everything holds together. Some of the bullets designed for extreme high velocity have thicker jackets and may do much better than expected. Jacket thickness being variable means there is no fixed spin limit for all bullets of the same caliber. Only those of the same caliber with the same jacket thickness.
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Old December 7, 2009, 03:18 PM   #6
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Thanks for the great information so far guys. Perhaps I should also be asking this question (particarly if anyone here has been firing 243's at long distance) Just what would be the optimal rate of twist for a bullet in the 105gr range? (or actually, the minuimal twist rate that I could get away with?)

And this one seems very important to me: IF I spin a light bullet so fast that it fails catastrophically, could it do so while in the bore and cause damage to either rifle or shooter?
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Old December 7, 2009, 04:02 PM   #7
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There have been cases reported of jackets coming off the bullet in the barrel, don't know if due to fast spin. That is catastrophic, because you still get regular recoil etc unlike a squib, but you end up with a barrel obstruction for your next shot. The infamous exploding Blaser R93 was claimed to be due to that.
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Old December 7, 2009, 04:20 PM   #8
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243

I have four custom 243s
Two with the 1-14 twist for shooting coyotes. I like the 55 grain bullets but even they like the 75 grain bullets better.
one of the rifles is 1-12 twist I shoot most all weights in it, except the 55 and real heavy ones. I get about 1.25 inch groups with the 107 grain bullets.
My last rifle was my sons a savage with a 1-10. Still the 107 didn’t do as well as the 100 grain, 5/8 inch group with H450. it would not shoot the 55 grain bullets at all.
I liked the 1-12 twist the best; with 75 grain bullets it bucked the wind well.
It worked well on coyotes also.
have fun and happy shooting.

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Old December 7, 2009, 05:04 PM   #9
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Check out:
http://www.shilen.com/calibersAndTwists.html

http://www.riflebarrels.com/products...wist_rates.htm
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Old December 9, 2009, 06:14 PM   #10
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300 Magman,

Bullet length has more to do with the right twist rate than weight does, which plays a smaller role. Check out the JBM calculators online. Look for Don Miller's calculator there and you can plug your bullets in. I have an Excel file based on Miller's method which includes some additional explanation of what you are looking for. You can download it here. You will need to know the length for any method of estimation, though. Often, the best way is to call the bullet maker's 800 number and ask what the average length is?
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Old December 9, 2009, 06:42 PM   #11
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This may be slight thread drift, but I was just wondering. Would a sold bullet like a Barnes hold together better than a copper jacket/lead core bullet at fast twist/high velocity levels? Doed anyone have any experience with these type bullets? Just curious. Thanks.
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