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What TWIST RATE for which bullet weight

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nralifer View Drop Down
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    Posted: August/25/2016 at 06:35
Since that experience if there is a question about stability we try and test in cold weather and for 500 yds or better.
It is hard to fail, but it is worse never to have tried to succeed. TR
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DCAMM94 Quote  Post ReplyReply Direct Link To This Post Posted: August/22/2016 at 14:10
I once had groupings like that (consistently) out of my old .243 Encore 15" handgun.  The only thing it would shoot consistently was 85gr, and I had to use compressed loads or the older hornady overcooked loads.  It was a 1-11" I believe.  Now I use a 7-08 barrel with a 1-9 twist, and it will hold a spiral a little better than a Billy Kilmer pass - at least out to 300yds - with up to 140grainers.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nralifer Quote  Post ReplyReply Direct Link To This Post Posted: August/17/2016 at 05:36
Thank you Peddler.It's a really interesting forum. Enjoyed Bigdaddy's  pic. Looks familiar. Have a great friend who likes to machine his own bullets for this monster 338 Lapua. He made a 325 gr machined copper bullet about as long as the Texas panhandle that actually shot sub minute of angle groups out to 3-400 yds. (not sure which one now). We wanted to test it at a longer range and so we set up a target at 500 yds. It was a a very cold day (5degrees,and pressure was 30.30 in Hg). Shot three shots, and all the holes looked like Bigdaddy's pic. The funny thing was the group was minute of angle!  We actually spoke to a Hornady ballistician about the phenomenon and he said it was not possible. Glad to see that someone else has witnessed the impossible as well.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bigdaddy0381 Quote  Post ReplyReply Direct Link To This Post Posted: August/16/2016 at 06:33


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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Post Options Post Options   Thanks (0) Thanks(0)   Quote Peddler Quote  Post ReplyReply Direct Link To This Post Posted: August/16/2016 at 05:15
Welcome to the OT George.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nralifer Quote  Post ReplyReply Direct Link To This Post Posted: August/15/2016 at 22:49
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: October/27/2012 at 17:17
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.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stevey Ducks Quote  Post ReplyReply Direct Link To This Post Posted: July/20/2012 at 01:52
1 - 11 twist for .308 diameter bullets 190 gr or more?
 
1-14 gr. for the real fast stubby ones.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 3_tens Quote  Post ReplyReply Direct Link To This Post Posted: May/10/2012 at 09:11
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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote River Runner Quote  Post ReplyReply Direct Link To This Post Posted: May/10/2012 at 05:18

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.

Randy
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snakeman48 Quote  Post ReplyReply Direct Link To This Post Posted: December/30/2011 at 20:20

Finally found this chart, again

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

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Post Options Post Options   Thanks (0) Thanks(0)   Quote xiaochun3612 Quote  Post ReplyReply Direct Link To This Post Posted: May/31/2011 at 23:57
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote budperm Quote  Post ReplyReply Direct Link To This Post Posted: March/17/2011 at 06:35
Yepper, its definitely one of the better ones!!!
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bigdaddy0381 Quote  Post ReplyReply Direct Link To This Post Posted: March/17/2011 at 06:31
this is the thread I was looking for.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote budperm Quote  Post ReplyReply Direct Link To This Post Posted: January/17/2011 at 09:42
Can a moderator Please Pin this post! Bandito
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Post Options Post Options   Thanks (0) Thanks(0)   Quote helo18 Quote  Post ReplyReply Direct Link To This Post Posted: January/16/2011 at 17:19
Thanks for the bump Wes. I was looking for that chart recently.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stevey Quote  Post ReplyReply Direct Link To This Post Posted: October/02/2010 at 00:26
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stevey Quote  Post ReplyReply Direct Link To This Post Posted: August/27/2010 at 01:38
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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kickboxer Quote  Post ReplyReply Direct Link To This Post Posted: August/03/2010 at 20:45

Great explanation, Tom...

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gunshow75 Quote  Post ReplyReply Direct Link To This Post Posted: August/03/2010 at 18:48

[/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. 


Tom



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