Excessive twist rate

S

Suwannee Tim

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What are the disadvantages of using a twist faster than necessary to stabilize the bullet? Particularly I ask in reference to ARs. I have several barrels in 1/9 and a couple in 1/7 and I can't tell the difference. The reading I have done indicates that faster twist increases barrel friction and heating, causes more deformation of the bullet which may affect accuracy, may increase pressure and causes the bullet to behave more explosively on impact. Any opinions?
 
A bullet at Sg 1.00 is considered stabile. Sg 1.40-2.00 is considered desireable. More twist than necessary may increase any effect of an unbalanced bullet and decrease accuracy. There are quite a few bullets that will perform well in a 1:7 to 1:9 twist.

You can check the Sg for a particular bullet and twist at: http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi - nhk
 
I have always vaguely wondered about this. My interest has been piqued by AR fanboys who condemn any barrel not 1/7. I have never had a problem with 1/9. I have not shot any bullet from 223 heavier than 70 grains nor do I see the point in such a bullet.
 
I've never shot a 1:7 barrel, but have had good luck with a couple of 1:9 AR's with 36 gr Barnes Varmint Grenades, 55 gr Nosler Ballistic Tips, and 69 gr Sierra MatchKings. The 1:7 twist is desirable for the heavier match bullets used in competition. There seems to be a gap in .223 barrels between 1:7, 1:8, 1:9 and 1:12, 1:14. - nhk
 
Aside from possible accuracy problems with unbalanced bullets, a twist that's too fast can cause light jacketed bullets like Sierra's Blitz, Hornady's SX, and Speer's TNT series bullets to be shredded. Most modern bullets are well enough balanced that a twist rate that's a little too fast doesn't hurt accuracy enough to make any difference except in BR competition.

A rough bore and too fast twist will almost be guaranteed to produce smoke trails toward the target rather than holes in it if the jackets aren't heavy enough. Don't ask how I know about this because I'll lie like a rug.
 
Suwannee Tim said:
What are the disadvantages of using a twist faster than necessary to stabilize the bullet? Particularly I ask in reference to ARs. I have several barrels in 1/9 and a couple in 1/7 and I can't tell the difference. The reading I have done indicates that faster twist increases barrel friction and heating, causes more deformation of the bullet which may affect accuracy, may increase pressure and causes the bullet to behave more explosively on impact. Any opinions?
Click to expand...

I will opine that for an AR platform the tighter twist is the better choice for these reasons:

-You will not be able to find the accuracy degradation in this platform. The accuracy loss is due to the higher rpm's exaggerating the effect of CG offset. The effect is too small to be mapped on an AR, lost in the noise.
-The tighter twist doesn't increase pressure or bore friction, no extra heat nor increased deformation.
-The tighter wound bullets WILL pop better on varmints IMO. All else being equal.
-I don't think the AR platform will blow up bullets unless you buy a really rough barrel.

These opinions are formed mainly from my personal experience.

al
 
To Alwina...

Quote...-The tighter twist doesn't increase pressure or bore friction, no extra heat nor increased deformation.

Is that in principle or just from a practical standpoint?
 
pacecil said:
Quote...-The tighter twist doesn't increase pressure or bore friction, no extra heat nor increased deformation.

Is that in principle or just from a practical standpoint?
Click to expand...

To paclice...

Only from a practical viewpoint, empirical anecdotal data.

al
 
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