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# Twist Rate and stability

#8
12-17-2013, 01:40 AM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 2,968
Re: Twist Rate and stability

Quote:
 Originally Posted by MontanaRifleman The plastic tip alters the specific gravity of the bullet and also pushes the CG back
Moving the CG backward is destabilizing though.
I've never understood why plastic tipped bullets are more stable than same form of non-tipped.

The Miller stability formula won't work with these.
#9
12-17-2013, 10:40 AM
 Silver Member Join Date: Apr 2012 Location: California Central Cost Posts: 345
Re: Twist Rate and stability

Quote:
 Originally Posted by Mikecr Moving the CG backward is destabilizing though. I've never understood why plastic tipped bullets are more stable than same form of non-tipped. The Miller stability formula won't work with these.
If I understand it correctly, the lower density of plastic does not generate a destabilizing mass forward of the CG. Takes less to 'go to sleep'.

Just grabbing the last JBM bullet flight times on my page.

~200,000 to 250,000 rpm
~20,000 to 25,000 revolutions in 100 yards. (.089)
~200,000 to 250,000 revolutions in 800 yards. (1.038)

Made me think about the going to sleep part. Like a thrown foot ball that starts a little wobbly then smooths out. I know this is a poor analogy but can't think of anything closer.
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#10
12-17-2013, 12:11 PM
 Platinum Member Join Date: Jan 2013 Location: Meridian, Idaho Posts: 1,099
Re: Twist Rate and stability

Quote:
 Originally Posted by bigngreen If you measure the metal part of the bullet from base to where the tip starts I can give you a number that take a polymer tip into account , bet your in the 1.2 range though.

Tip to base is .75. I have not measured without the tip. .68 is a guess.
#11
12-17-2013, 12:19 PM
 Platinum Member Join Date: Nov 2008 Location: SW Montana Posts: 4,201
Re: Twist Rate and stability

Quote:
 Originally Posted by BrentM Tip to base is .75. I have not measured without the tip. .68 is a guess.
That puts you at 1.12 Stability Factor when figuring in the poly tip. Bottom line if they shoot, they shoot!
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#12
12-17-2013, 01:59 PM
 Platinum Member Join Date: Jan 2013 Location: Meridian, Idaho Posts: 1,099
Re: Twist Rate and stability

What program are you using? Shooter and Berger are lower, I realize it doesn't matter that much if they shoot fine. Just trying to decide if this factor is even worth looking at any more. I using like to look at it for fun and listen to people discuss the subject about over or under stabilizing the bullet.

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