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7mm 175-180 SMK twist rate

 
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  #1  
Old 01-29-2011, 10:03 PM
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7mm 175-180 SMK twist rate

Sorry for starting another thread about bullets, but hey that's what this section is here for right?

I've been obsessing over the 7mm 180 Berger Hybrids in my 1:9.5 twist barrel, and finally shot some today. They stabilized out to 300 yards just fine, but I'm a little worried that when the temps drop and the air gets heavy these might have a problem stabilizing.

Anyways... I got to looking at the 7mm SMK's and was running the JBM calculator for stability on these bullets. It seems that the 175's will stabilize fairly easy out of a 9.5, but Sierra recommends a 8.5.

To my understanding length has alot to do with stability.
168 Berger VLD (1.445) recommended twist 1:10
180 Berger VLD (1.525) recommended twist 1:9
175 Sierra Matchking (1.453) recommended twist 1:8.5
180 Sierra Matchking (???) recommended twist 1:8
168 Sierra Matchking (1.365) seems to be the most popular for factory twist

The 175 is just barely longer than the 168 VLD, and quite a bit shorter than the 180 VLD, so I'm confused as to why it would need more twist than either of the VLD's?

I really want to stay above .6 on the BC with whatever 7mm bullet I shoot, but this whole stability thing has got me all messed up.

Last edited by jsimonh; 01-29-2011 at 10:36 PM.
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Old 01-29-2011, 10:33 PM
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Re: 175-180 SMK twist rate

What Caliber are you talking about 175-180 SMK leads me to believe 30 Cal. but your numbers are way off. So I was figuring you meant 175 SMK - 180 Berger 7mm?
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Old 01-29-2011, 10:35 PM
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Re: 175-180 SMK twist rate

Quote:
Originally Posted by Rimfire View Post
What Caliber are you talking about 175-180 SMK leads me to believe 30 Cal. but your numbers are way off. So I was figuring you meant 175 SMK - 180 Berger 7mm?
7mm, sorry for not being clear.
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Old 01-29-2011, 10:49 PM
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Re: 7mm 175-180 SMK twist rate

I've had great accuracy with the 175 smk in a 9 twist 7wsm
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Old 01-30-2011, 01:19 PM
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Re: 7mm 175-180 SMK twist rate

I know the 180 SMK uses a secant ogive like the VLD's, but does anyone know what the 175 uses?
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Old 03-23-2011, 07:25 AM
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Re: 7mm 175-180 SMK twist rate

Bump ... interested too.
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Old 03-23-2011, 08:37 AM
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Re: 7mm 175-180 SMK twist rate

Quote:
Originally Posted by jsimonh View Post
I know the 180 SMK uses a secant ogive like the VLD's, but does anyone know what the 175 uses?
If I recall correctly, the 175 was a tangent ogive. Sierra uses the tangent as a standard default in their bullet designs, and the secant ogives in their line are the exception, not the rule.

Wouldn't worry so much about the twist issue, either. Sierra tends to be extremely conservative in their twist recommendations, and you can often get away using something a bit slower than what they suggest. Berger, on the other hand, gives you just about what's needed for a minimum stability factor of 1.3-1.4, which is what you want.
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