Thread: Twist rate vs velocity vs bc

So for any given bullet in theory for the same pressure more velocity should be obtained by a slower twist using less energy and friction to rotate the bullet, what are your experiences with this?

An over stabilised bullet may not tip and follow its arc of travel but stay spinning on the axis of the bore at long range increasing drag as it falls threw the air, anyone observed this?

How does the stability factor affect the way the bullet transitions the sound barrier at ELR or is this more a function of the form of the bullet?

Ohhhhhh, good thread!!

The theory is solid but I have not seen any measurable difference that I could attribute solely to the twist rate.

I guess you would have to shoot the same projectiles through the same barrel but somehow changing the twist rate (obviously impossible) to see the differences?

Also, I would have thought the projectile that was still spinning the fastest as it dropped transonic would be the most stable. All things being equal.

Interesting questions. I have never been able to properly understand the problems of transonic change at long range. When I used to be involved in NRA competition we used ex army .303 service ammo and shot out to 1000 yards. I never heard any of the old shooters complaining about bullet instability and when working in the butts I seldom saw bullets go through targets sideways. These bullets were 174 grain flat based. Maybe they somehow managed to stay supersonic because the sound of the bullet going overhead at 1000yards was usually a hollow "pop", and yet the rated Mv was only 2440 feet per second. I don't know what the twist rate was in the SMLE No 3 and 4 barrels. Some were only two groove and one I had like this in No 4 was very accurate with a standard service barrel.

The better shooters could consistently put most of their rounds through a 30 inch bull at 1000yards using aperture sights and estimating windage corrections from flags down range.

I will be interested to see what develops in this discussion.

This thread might answer some of your questions Nosler LR Accubonds: BC testing results - Long Range Hunting Online Magazine Brian Litz tested the new Nosler Long Range Accubonds and found that a faster twist increased the BC for for the projectiles he tested, but sometimes not by much. Not sure if he kept velocities or powder charges the same.

A very interesting thread. I like what Woody has to say about the .303.

My 10c

I loaded up some 208 amax for my mates Omark the other day. Some 2206H, seated long...... still only managed about 2400fps from a 1:14 twist 24" barrel...obviously not going to stabilise according to everything I thought I knew....

That load grouped sub moa right out to 600m....I couldn't believe it. We haven't tested past there yet.

Originally Posted by veitnamcam

So for any given bullet in theory for the same pressure more velocity should be obtained by a slower twist using less energy and friction to rotate the bullet, what are your experiences with this?

An over stabilised bullet may not tip and follow its arc of travel but stay spinning on the axis of the bore at long range increasing drag as it falls threw the air, anyone observed this?

How does the stability factor affect the way the bullet transitions the sound barrier at ELR or is this more a function of the form of the bullet?

In Bryan Litzs latest book he says the difference in velocity from slow to fast twist is typically less than the usual ES so basically no difference.

Also a faster twist on a bullet gives it a slightly higher BC than a slower twist, so that would probably cancel out or over ride any loss of velocity at distance.

It is possible that over stabilised projectiles can nose up causing a loss of BC at extreme range.

A client has tested my .375 425 CEB projectiles in my old 7.25 exit 375 DCM at 4000m

They would have been way subsonic but they were still stable & nose up even with the 20-25 degree angle of decline when they hit the target

Even though they were nose up they still went transonic fine & the trajectory was repeatable.

I'm really looking forward to testing of the 400 & 425s in my 40" 1-9 twist 375 DCM & my 452s in the 7.25 exit twist 375 DCM, should be interesting.

It doesn't really matter what is "supposed" to happen in theory, what matters is what happens in practice, like the 208s in the 14 twist in a previous post

Originally Posted by Kiwi Greg

In Bryan Litzs latest book he says the difference in velocity from slow to fast twist is typically less than the usual ES so basically no difference.

Also a faster twist on a bullet gives it a slightly higher BC than a slower twist, so that would probably cancel out or over ride any loss of velocity at distance.

It is possible that over stabilised projectiles can nose up causing a loss of BC at extreme range.

A client has tested my .375 425 CEB projectiles in my old 7.25 exit 375 DCM at 4000m

They would have been way subsonic but they were still stable & nose up even with the 20-25 degree angle of decline when they hit the target

Even though they were nose up they still went transonic fine & the trajectory was repeatable.

I'm really looking forward to testing of the 400 & 425s in my 40" 1-9 twist 375 DCM & my 452s in the 7.25 exit twist 375 DCM, should be interesting.

It doesn't really matter what is "supposed" to happen in theory, what matters is what happens in practice, like the 208s in the 14 twist in a previous post

Cool! so in the above example bullets were seen to be staying on the line of bore presumably by shooting at cardboard/paper?

Originally Posted by veitnamcam

Cool! so in the above example bullets were seen to be staying on the line of bore presumably by shooting at cardboard/paper?

I haven't seen the targets but apparently there is a nice round hole with a wee bit out of the top made by the tip.

These bullets are extreme, they are 2.312", 58.72mm long with a BC of 1.03

Also if you look at some of the most accurate cartridges in the North American Benchrest circuit, the 6 mmbr and dashers perform pretty well from 600 to 1000yrds. They are not shot at crazy velocities , but the twist is often 1 in 8" for those above 100 grainers pills typically used. So the deformation of the jacket theories does not seem to apply here. The early 7 mm 162 grain a-max were notorious for vaporising from time to time in fast twist and fast speed cartridges. I believe hornady has sorted out the problem since .

I kind of wonder whether some of this bullet stability stuff is hype. The old Vickers (and other machine guns in .303 cal used be be set on a set firing line to cover from a very long distance.

The precision shooting competitions in USA seem to be out to 1200 yards with little 6mm weapons.

Is the stability concern mainly only applying to extreme accuracy requirements at extreme range. I.E over 3000m or so?

The world record sniper kill was about 2000m plus, I believe. How did bullet stability come into that?

Maybe military bullets are inherently stronger and more stable even when dropping through the sound barrier.

I.E. How much is sales hype and how much is real?, and are some of the modern bullet designs actually not that great?,, while others, like the tried and trusted Sierra's are older verywell proven designs which function well at all ranges.

The link that Stug posted had some very interesting reading in it.

Basically a bullet that is spun fast enough to be "stable" will fly wobbling around its axis slightly increasing drag compared to one spun faster than the minimum that will spin more true.

This is not an accuracy concern as such until you account the extra windage from the only just stabilised one being blown further off course at long range and the lower impact velocity if shooting at living things.

One thing i do remember with barrel twists from reading a while ago on the 22-243 is that you can over spin the projectile fast enough as when they get the full drag through the air they can break apart and vaporise.

Im sure it was a thread buy KiwiGreg or crazyman

I found it very interesting goes to show that you want the correct twist for bullet and velocity as shown at the extreme end.

Originally Posted by 199p

One thing i do remember with barrel twists from reading a while ago on the 22-243 is that you can over spin the projectile fast enough as when they get the full drag through the air they can break apart and vaporise.

Im sure it was a thread buy KiwiGreg or crazyman

I found it very interesting goes to show that you want the correct twist for bullet and velocity as shown at the extreme end.

My Ex Cryzman 26", 1-7, 22-243 shooting the 90 Bergers at 3100-3300fps is extremely accurate.

The trouble starts when the barrel gets warm, the warmer it gets the lower the velocity threshold is when the projectiles vaporise.

The heat build up going down the barrel & through the air melts the lead between the jacket & core, the bullet gets unstable & disintegrates between 50-100 yards out.

I had a "butterfly" hole in one of my melamine targets once when the projectile let go directly in front of it at 100 yards.

At 31-3150fps you can shoot around ten rounds in summer before its an issue, at 3300 one or two if you are lucky.

HA! I knew I'd find this. Its an unbelievable still frame a fella caught of a .338 a fraction of a second before impact. Look at that angle that she's coming in on. He was shooting out to 2500m but its interesting that his projectiles aren't nosing in.



http://www.liveleak.com/view?i=a53_1400252462

comeing into summer owning a 22-250AI running 75 GR A-max at 3450 i am at a similair experience to kiwi greg. as for twist rate there is two main concerns when useing a projectile lighter than intended for the twist

1. jacket core seperation / bullet blow-up, at extremely high RPM's this can happen with match and varmint projectiles but with premium bonded core projectiles or full copper variants are less likely to experience this scenario if at all.

2.velocity chokeing / skipping of rifleing, if this occurs it can effect accuracy and performance, although for this to occur you would need double the reccomended twist at near over-bore velocitys. as for velocity loss my experience is that there is no obvious change in velocity with twist rates exceeding the reccomended by 30-40%.

as for how excess rotational force effects the projectile at longer range's, in particular the transonic region is not something i have a depth of knowledge on myself. what i have observed is that under 300 yards most projectiles at high velocities have quite unstable rotation much like a rugby ball as it is first thrown it is quite unstable ( or in fancy speed pitch's and yaw's in an oscillatory manner https://www.youtube.com/watch?featur...&v=4pF8W5liSRc ), but at a certain part of its flight path it progresses into a stable rotation. a term used for a projectile as "going to sleep".

which is why i do not devote all of my development only to the 100m range as i have found that alot of loads that i have not deemed as being the most accurate ( although really close) have turned out to ruduce grouping size at 300m. anyway ill leave it at that before i go too offtrack if that hasnt happened already hope some of this is useful or atleast some what worth reading.