What TWIST RATE for which bullet weight

note -- one of the inputs is length

yes the military uses the smk for the long range load. the bc is still only around .4 so wind drift is still a factor, especially when the load is shooting below 3000 fps.

Great chart Wes... I was just looking for a cross reference chart like this and walla!  I couldn't find the link online for it.  I captured the chart image in a .jpg file if anybody would like it so they can just print the chart off. 

uploads/4389/twistrate.zip

Edited by scooter65 - January/24/2010 at 12:46

lilja barrel company has a very nice chart as well, i have a copy of it printed out that i keep near my computer so when im looking at rifles and potential bullet combos in those rifles i can look at the twist rates and decide if its worth a shot or not.
http://www.riflebarrels.com/products/caliber_twist_rates.htm


Edited by pyro6999 - January/24/2010 at 11:53

pyro6999 wrote: lilja barrel company has a very nice chart as well, i have a copy of it printed out that i keep near my computer so when im looking at rifles and potential bullet combos in those rifles i can look at the twist rates and decide if its worth a shot or not. http://www.riflebarrels.com/products/caliber_twist_rates.htm

Berger Bullets website lists the recommended twist rate for every bullet they make.

Dale Clifford wrote: note -- one of the inputs is length http://www.jbmballistics.com/cgi-bin/jbmstab-5.0.cgi

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?

I'm going to get some bulk Federal Gold Medal Match in .308 Win...my Ruger is 1:10 twist...Which would suit my rifle best...168gr?...175gr?...or both???

Normally the faster twist will stablize a heavier bullet.  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 your question so if I had to say one of the two it would be the 175 because of wind drift

 Anyone familiar with this:

Interesting link Earl!  I will have to save it and study it when I have more time!

  Worked for me when I had to decide what rate of twist I wanted my 9.3 rebored to. Ended up at 1-12.

trigger29 wrote: Dale Clifford wrote: note -- one of the inputs is length http://www.jbmballistics.com/cgi-bin/jbmstab-5.0.cgi 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?

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.

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

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.

What did he just say?!?!?   I think he is a closet lawyer.

 

 

I followed some of that but most went over at a rather high altitude.

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.

[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. [/QUOTE]

ask the question from a different view-- what drag forces are acting on the rotational spin?

i need some aleve

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.

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

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..