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What TWIST RATE for which bullet weight

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/20/2010 at 20:22
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here is a good article if interested.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/20/2010 at 21:00
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Originally posted by pyro6999 pyro6999 wrote:

Get Your Popcorn Ready i need some aleve
.
 
 
 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/20/2010 at 21:03
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you got it D!
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: July/30/2010 at 02:04
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SmileThe Sierra manual has some charted data showing the effects of twist rates on bullets. For example, a chart shows the effects of twist rates on their 69 grain .224 MK affecting measured ballistic coefficents. This chart shows a 1-7 twist results in the most uniform b.c. 1 in 12 twist makes for b.c.'s that not uniform.
 
The 190 grain .308 MK seemed to like 1-10.
 
As the projectile is launched out of the barrel a number of factors come into action such as yaw and precess (sp?). Play around with a gyroscope and push it to one side and it will move at right angles -- in a bullet this is called "coning". Anyhow, if I want one hole groups at 100 yds I shoot short bullets with a slow twist. If I want to hit far away targets when the wind is blowing I shoot long pointy bullets that need a fast twist. I cannot remember shooting tiny groups with my 6.5-06 using 142 Si MK's (1-8 twist).
 
The Sierra manual has numerous diagrams that show what happens when a bullet is launched down range (force and direction or vectors) - terms such as yaw, deflection, center of mass, center of pressure are described. The diagrams show what occurs. An anaysis of the situation using first order dif equations or thermodynamics would bring me back to my class room days when I could not afford rifles and had no time for shooting.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/01/2010 at 23:40
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SmileIs obduration the increase of the base area of a bullet caused by expanding gas pressures caused by burning gun powder? My guess would be that most jacketed rifle bullets would have zippo obduration but civil war era mini ball type projectiles would have the most. Miking my boat tail jacketed rifle bullets dug out of snow banks seems to indicate no change from bullets out of the box, except for rifling marks, from the snow bank bullets.
 
I would guess that shooting tiny groups such as in bench rest competion is somewhat of an art and maintaining a small variation in ballistic coefficients by selecting the proper twist rate is only part of the problem. Bench rest competion at 100 yards with a 6mm PPC with 75 grain bullets would be different than bench rest shooting at 1000 yds with a 6.5-.284 with 140 -142 VLD bullets. Switch rifles for each event and realize a zippo score.
 
If in doubt go for the faster twist.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/02/2010 at 08:48
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It occurs because of the differences in the moment of interia within the body of the mass. The more mallaeble the material the more obtruation. In softer lead bullets the base is accelerated faster than the front  filling in the lands. If the acceleration is hard enough the bullet will strip instead of spin and lead is left in the barrel. Gas checks are added to stop this and gas blow by. All real life stuff when accelerated exhibit set back. Sometimes its better study these under conditions called elastic collisions, when some of the conditions are relaxed. (atomic particles) Even soft lead bullets when recovered in a snow bank will show no differences when miked out.
most shooting sports are the antithesis of bench rest shooting. In real life shooting very little observable differences occur when the bc of bullets only varies by a factor of .05. Faster twist barrel will always have the trade of more wear and lower velocity.
There have been quite a few long range records set with non boat tail seemingly high drag bullets, and I shoot sub moa groups all the time with boat tails (as do others) at 100 yds. Most of the differences in bench rest come from the different guns, and its a reloading sport as much as a shooting sport.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/02/2010 at 12:10
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Hey, what about .458 and .510????  
I see some discrimination here...
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/02/2010 at 12:20
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Not at all --- long range shooters at Raton NM use 45 sharps etc. for 1000 yds and longer and none of their bullets are pointy.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/03/2010 at 18:48
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[/QUOTE] Dale, I noticed that the stability factor changes with velocity. We all know that bullets don't stay the same velocity after they leave the barrel, so I was wondering, if your velocity is 2900 with a stability factor of 1.135 What happens when the bullet gets to 1900 fps, and the stability factor drops to .986. Will the bullet destabilize, and start to tumble or wobble? [/QUOTE]

I assume you are inquiring about the stabilty factor for the bullet at a point down range when it has slowed to 1900 fps.  If so, the stability factor does not get smaller; it is actually larger.   
 
I think you are discussing the gyroscopoic stability factor (GSF) at the muzzle.  If we define the stability factor at the muzzle to be S, the more general equation for the stability factor is S * (Vm/V)^2, where Vm is the muzzle velocity and V is the velocity at any point down range.  The stability factor is lowest at the muzzle, and that is where the value for the twist is determined.
 
Since there were also inquiries about bullet RPM, the general equation is also proportional to spin rate.  Spin rate decreases as the bullet goes down range, but it decreases much more slowly than the bullet's forward velocity decreases. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/03/2010 at 20:45
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Great explanation, Tom...

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/27/2010 at 01:38
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SmileYes, boat tail bullets can shoot sub MOA @ 100 yds but bench rest matches require better than that. A 70 gr 6mm PPC can kill small bugs at 100 yds.  A 45-70, 300-500gr. can make it out to 1000yds but at that range it has been sub sonic for almost half its flight (blooby trajectory). Help me out on this ... is Hookes law, as pertaining to the elastic qualities of metals ? (bullets in snow banks) causing the bullets to contract after being obdurated by either expanding gasses (rapid pressure increase) or differences in acceleration forces applied to the end of the bullet as opposed by the other pointy end that is at rest at the start of acceleration from say 0 to over 3000 fps. Is a stubby mini ball with its hollow base puffed up by a blast of black powder? Has anybody ever seen signs of gas blow by from firing a tightly fitting jacketed rifle bullet through a precisely matched rifle bore? My guess is they either slide through cleanly or get stuck -- possibly there is no room for obduration.
 
If in doubt go for a faster twist. 
My feeling is that B.C.'s vary with velocity as much as 10% (or possibly more) due to primarily coneing and any effect due to obduration is zippo as acceleration stops after the bullet is launched and providing the elastic properties of the bullet are not exceeded the bullet assumes it's prelaunch diameter except for rifling and other marks.
 
I have 2, 1-8 twist .243's that shoot 107 & 105 gr boat tail bullets just fine -- even up to 1000yds but I have not been able to hit a small bug at 100 yds with either of them. I would guess a 1-8 twist would be marginal with 115, 6mm's and 1-7 would be better  -- the observation of keyhole's at 100 yds and not at 50 yds is of note, I guess things just keep on going bad down range.
 
 I have noticed the groups fired with 142 gr SiMK (1-8 twist) at 100 yds are only moderately better than those fired at 300yds. Is there any truth to the statement that long pointy bullets "go to sleep" at some extended range.
 
Are rebated boat tail bullets still sold? My guess is that there is no advantage to them as obduration is not a factor
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/02/2010 at 00:26
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SmileOn the obduration question -- at say 50,000 to 65,000 psi anything that can be deformed or squashed will be obdurated and accelerated (progressive gun powders). Acceleration is a result of a force rather than causing a force.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/16/2011 at 17:19
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Thanks for the bump Wes. I was looking for that chart recently.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/17/2011 at 09:42
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Can a moderator Please Pin this post! Bandito
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/17/2011 at 06:31
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this is the thread I was looking for.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/17/2011 at 06:35
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Yepper, its definitely one of the better ones!!!
 
Good help to us all not just newbies!
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/31/2011 at 23:57
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I would try both to see which provided better accuracy, if they are equal then get teh 175. It will fight the wind better than the lighter grain.  With a 10 twist you may even be able to put some bullets up to 200gr or even the 208 hornady a max.  I know this doesn't really answer.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: December/30/2011 at 20:20
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Finally found this chart, again

Been lookin' for it, found it, then lost it.

Thanks
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/10/2012 at 05:18
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So if I know the twist rate of the barrel on my gun, should I choose a bullet weight based on that twist?  And is the twist rate easy to find out?  I have a Winchester model 88 in .308.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/10/2012 at 09:11
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Each barrel has different dynamics. Even 2 barrels that are made by the same machine one right after the other will shoot different. Learning your weapon comes from careful notes maintained over time. Then about the time you get it all figured out its time to re-barrel the rifle and start all over again. Wink
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: July/20/2012 at 01:52
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1 - 11 twist for .308 diameter bullets 190 gr or more?
 
1-14 gr. for the real fast stubby ones.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/27/2012 at 17:17
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GOOD Thread BUMP even though it is a sticky icky...

I have found limits as far as speed and weight that are factors as well as barrel material and heat of barrel and specific heat properties a factor as well as bullet material hardness and coef rise.

BUMPED
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/15/2016 at 22:49
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I thought I might post something on this thread since there has not been one in a long time.  The subject of stability and barrel twist rate is something that I know a little about since I have had to deal with it a lot.  Stability is a function of bullet length, caliber muzzle velocity, air density and all the factors that influence that, and bullet weight.  The simplest thing to do is to go to the JBM Ballistics site and play with their bullet stability calculator.  To be sure that the bullet one wishes to try will be stabilized by a barrel of a given twist simply measure with a calipers the length of the bullet, weigh it, and plug in the numbers into the JBM calculator. For atmospheric variables one can use standard conditions such as 59 degrees F, 29.92 in Hg, and 50% humidity. For a bullet to be stable under most conditions, a stability factor of at least 1.4, and preferably 1.5 is needed.  As the atmospheric pressure increases and the temperature decreases, the air gets denser and the stability factor decreases. A bullet, when shot in the summer, may appear stable, but come winter time the stability may not be there (hit sideways or shoot bad groups on cold days).  When the stability factor under a given set of conditions falls below 1.25 then weird things can happen like it is accurate at 200 yds but key holes at longer ranges.  Also it will have a tendency to tumble through the target and have erratic penetration through an animal since flesh is denser than air.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/16/2016 at 05:15
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Welcome to the OT George.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/16/2016 at 06:33
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This is my best load to date. I like to use the side of the bullet for a better energy dump. The bullet dose not mushroom out like some I have seen, but they do have good knock down power. To get this fine groups at 200 yards I have to use a slower twist and heaver bullets.s This is 200 yard 5 shot group.

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