Twist rate

[Nick 53]

Hi all what twist rate determines slow and fast ? And how’s is the bullet deemed suitable for that twist rate?

 Cheers Nick 

[VarmLR]

The required gyroscopic stability of a bullet is related to selection of twist rate, or rather the bullet's stability factor (related to gyroscopic stability) determines whether it will be satisfactorily stabilised for any given twist rate.  Brian Litz has published extensive research on this and his first publication of Applied Ballistics for Long Range Shooting, as well as his latest one provides tables which list the Sg (stability Factor) for a whole range of bullets and calibres.  A nominal Sg of 1.4 or greater is the desired minimum figure to look for when seeing if a particular bullet might stabilise from  specific barrel twist.

Stability can be described in general terms as the ability of a projectile to maintain its point forward orientation in flight and return to that point forward orientation if disturbed (Litz, Applied Ballistics Ch10).  The only way that a smooth projectile like a bullet can do this is by spinning in flight.

The measure for stability is the Stability Factor, given by the ratio of the rigidity of the spinning mass divided by the overturning aerodynamic drag.   This then relates to bullet length and twist rate for any given calibre. An Sg of 1.0 or less will be very unstable.  1 to 1.4 will be marginally stable and anything above 1.4 will be stable.  However that is just a guide and there may be some leeway either side.

However, that is not to say that shooting a light short bullet from a fast twist will always be satisfactory as the high acceleration and velocities reached for these lighter bullets and the corresponding centrifugal forces can combine within the barrel and structurally compromise the bullet's outer jacket causing it to disintegrate.

The subject of stability leads onto some interesting related issues of trajectory which, whilst rarely an issue for small arms shooters can become a consideration for artillery pieces (over stabilisation affecting a projectiles ability to follow a trajectory trace, more importantly concerned with whether the projectile lands "nose in" to detonate at all due to maintaining a nose up flight attitude) but that's not something that really concerns sport shooters, although there may be slight effects on drag, hence BC with range/velocity.   Such issues as increased drag can be determined by field testing and calibrating DOPE accordingly.

This is a simplified explanation.  Laurie's really the man to offer more insight.

[Nick 53]

Thanks 

[gbal]

Nick-very basically,for any given calibre,as bullets increase in weight (length),they need a faster twist to stabilise-Varm gives some 'why' detail-there is 'tolerance' to some extent-generally this is better going lighter-the faster twists  will be ok (if not quite ideal) with lighter bullets,but the slower twists just won't work with heavier bullets.

If you just want to see what bullet  weights 'need' particular twists,then either the Berger Stability calculator...or the advice on Accurate Shooter (223 cartridge).

[1234]

Is there a way to easily confirm the twist rate in my barrel please?

[Bianchi]

A cleaning rod with a patch or VFG felt tip installed . When the patch is beyond the breech, mark the cleaning rod, such as at the start of the rod guide. Continue slowly pushing the rod through toward the muzzle until the mark has rotated 360 degrees (a full turn) and mark the rod again.

The distance between both marks should be the barrel"s twist unless the barrel (some Bartlein I believe) has a progressive twist rate. Repeat to verify.

Easy as 1,2,3....  .

[1234]

That sounds logical - thank you 

[Nick 53]

23 hours ago, gbal said:

Nick-very basically,for any given calibre,as bullets increase in weight (length),they need a faster twist to stabilise-Varm gives some 'why' detail-there is 'tolerance' to some extent-generally this is better going lighter-the faster twists  will be ok (if not quite ideal) with lighter bullets,but the slower twists just won't work with heavier bullets.

If you just want to see what bullet  weights 'need' particular twists,then either the Berger Stability calculator...or the advice on Accurate Shooter (223 cartridge).

Hi George got Berger stablizer calculator will take a look.

[Nick 53]

Just had a go on Berger stablizer calculator 

put in 105VLD MV 2800 1-8 

the calculator gave BC 0.265 full stability 

i only guessed on MV 2800 

 

 

[gbal]

NIck- play by all means,but bear in mind that all these ballistic machines tell it more or less how it is (ie assume the input data is accurate),and  poor data in means poor result out!

Entering 105VLD -from the given list- means 6mm (eg 6BR)...and the BC will be entered as input too;the guessed but input velocity of 2800 is low for a decent 6mm (a 6br eg could be 3000),but anyhow- input  that and twist 12,or10 -and it will predict 'unstable'- input 8 and  its stable (stable of course isn't an absolute,and varies).  But remember as described,this is all predicated on the input data-though all here is achievable,it may not all be very plausible/real world.

   Calculator is meant to give a stability assessment-given actual real firing velocity data,and actual rifle twist,for the bullets offered (or accurate data on any other used/entered).

But play a bit (call it ballistic modelling,if you aspire to rocket science)  and you will see how critical twist can be (best stay with the list bullets,otherwise too much guessing (BC and MV eg) and plausible MVs...and  when used sensibly,it is quite informative and interesting,and can  help avoid expensive errors.

 

g

[Nick 53]

Yes I will play, but I have No intentions to mess . I am to dam simple to mess with physics that are beyond my capacity.