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What TWIST RATE for which bullet weight

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Bigdaddy0381 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bigdaddy0381 Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2010 at 08:07
Originally posted by Dale Clifford Dale Clifford wrote:

while  the bullet decreases in speed, the rotational rate stays the same, so the amount of revolution per unit of distance traveled remains the same it takes more time to do it at the slowed velocity rate. Upon firing the base of the bullet is accelerated faster than parts of the bullet above this, (obtruation sp?). the more weight in the back causes better bc, but also gives more of shuttlecock effect. HBWC are an extreme example. At some point a bifurcation or change of a parameter in the relation between the inertial masses  of the front and back and the tumbling occurs. Pitch and yaw are different.
 
So a 1-10 twist stays 1-10 regaurdless of fps?
 

Dose obturation effect the flight of the bullet?

 

 
 


Edited by Bigdaddy0381 - May/19/2010 at 08:16
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dale Clifford Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2010 at 11:17
the spin rate doesn't slow down like the fps rate (more accurately -- the differential rate of change is considerably less, and the time or space constant can be ignored to linerize the problem). Obturation sets the conditions, if the bullet doesn't seal, gas blow by will tip the bullet on exit.
Especially true with cast bullets and alloy types. Yaw and pitch are oscillations or periodic sine waves that occur when position and time are plotted that occur. Bifurcation happens when the center of mass is moved thru the axis of the bullet and at some point the system falls apart and into an "attractor" of more stability.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote budperm Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2010 at 11:28
What did he just say?!?!?   I think he is a closet lawyer.
 
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dale Clifford Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2010 at 11:42
Numerical analysis can have different forms. When its used with ballistics as we talk about the calculations are done with numerical integrators using first order linear differential equations, that when plotted against each other, (the first matrix is time, the second is trajectory, etc, depending on the variable input) gives the plots and figures seen in places like JBM and Sierra. Usually a system (control systems) methodology isn't used because it gives other types of information, (such as the above) which doesn't really help someone hit something. A similar example is thinking of bullet energy (in terms of classical mechanics) as opposed to thermodynamics. Each give different information because the approach is different.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bigdaddy0381 Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2010 at 11:47
[QUOTE=Dale Clifford]the spin rate doesn't slow down like the fps rate (more accurately -- the differential rate of change is considerably less, and the time or space constant can be ignored to linerize the problem). Obturation sets the conditions, if the bullet doesn't seal, gas blow by will tip the bullet on exit.
Especially true with cast bullets and alloy types. Yaw and pitch are oscillations or periodic sine waves that occur when position and time are plotted that occur. Bifurcation happens when the center of mass is moved thru the axis of the bullet and at some point the system falls apart and into an "attractor" of more stability.
[/QUOTE]
Good point I didn't think of it that way. I was thinking more of the base of the bullet would go to one side or the other and not have a even base/contact. like 40% to 60% bottom would make the bullet wobble when it slows. Like out running a tire wabble on the high way. feel it slow and you don't when you speed up.
 
At what rate do you think or see it slowing too? If I can find the email from a controled test I'll post it. from the test it showed the rotation slowed as the velocity slowed and the twist stayed the same. Let me do some digging..
 
 
 
 
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dale Clifford Quote  Post ReplyReply Direct Link To This Post Posted: May/19/2010 at 17:00
ask the question from a different view-- what drag forces are acting on the rotational spin?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote trigger29 Quote  Post ReplyReply Direct Link To This Post Posted: May/20/2010 at 07:36
It seems to me that if a bullet is stabilized when it leaves the barrel, it should stay that way as long as no outside force interrupts it. The bullet's velocity will bleed off fairly fast, where as the rotational force should not lose it's speed nearly as fast...........Or I'm way off base, and as usual don't know what I'm talking about.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bigdaddy0381 Quote  Post ReplyReply Direct Link To This Post Posted: May/20/2010 at 08:23

The twist has to slow some. The ojive has a bearing surface that has an extreme force pushed upon it. I agree it will not slow as fast as the velocity but it slow to where the twist rate stays the same. If it didn't the twist rate would speed up. Say a 1-10 at 3,000 fps @ 20yards and a 1-5 at 1,500 @ 800yards if the twist didn't slow when shot from the same rifle. It’s easier to say than type.

 



Edited by Bigdaddy0381 - May/20/2010 at 08:26
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cyborg Quote  Post ReplyReply Direct Link To This Post Posted: May/20/2010 at 10:37
Air has a mass, and as such forces friction upon things moving whether they are spinning or moving in a path. That affect is a constant. The bullet will lose spin at a proportional rate that it loses forward movement so long as it remains in flight..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dale Clifford Quote  Post ReplyReply Direct Link To This Post Posted: May/20/2010 at 20:22

here is a good article if interested.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sgt. D Quote  Post ReplyReply Direct Link To This Post Posted: May/20/2010 at 21:00
Originally posted by pyro6999 pyro6999 wrote:

Get Your Popcorn Ready i need some aleve
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pyro6999 Quote  Post ReplyReply Direct Link To This Post Posted: May/20/2010 at 21:03
you got it D!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stevey Quote  Post ReplyReply Direct Link To This Post Posted: July/30/2010 at 02:04
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stevey Quote  Post ReplyReply Direct Link To This Post Posted: August/01/2010 at 23:40
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dale Clifford Quote  Post ReplyReply Direct Link To This Post Posted: August/02/2010 at 08:48
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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kickboxer Quote  Post ReplyReply Direct Link To This Post Posted: August/02/2010 at 12:10
Hey, what about .458 and .510????  
I see some discrimination here...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dale Clifford Quote  Post ReplyReply Direct Link To This Post Posted: August/02/2010 at 12:20
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.
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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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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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