Optimal/ ideal stability factor?

[DJ Fergus]

I know that this subject has been beat to death but I'm just wanting some feedback to verify whether my thoughts & experiences on the subject are valid. Based on the Miller stability factor a value of 1.5 is needed to fully stabilize a bullet. Beyond 1.5 stability, b.c. seems to only increase slightly. We know that stability doesn't necessarily mean consistent accuracy but on the other hand, stability is needed for consistent long range accuracy. I dug up some info from years ago & I can't remember who stated this info but it was from two fellows who were professional experts at the time. They stated that between a stability factor of 1.4 -1.7 based on the Miller formula is where the best accuracy would be found. I also know that Bryan Litz stated that there were benefits to staying below a stability of 2.0. I have also heard that rifle torque with faster than necessary twist rate rifles could be a potential problem, but is not always the case. Several others I have talked with have also told me that load development can become more finicky if theres too much twist for a particular weight bullet and I believe there is some truth to that.
All this being said, I have a 7.5 twist 6.5 creedmoor that starts to struggle below 140 grains and the lower weight bullet I go the more finicky load development gets. It will absolutely not shoot a 100 grain handload with accuracy. I have had the same issue with a 9.25 twist 7mm rm. Load development was a chore with 140 grain bullets. Load development was much better with 162 grain bullets & even better with 180 grain bullets.
Likewise, I am fully experienced with loading heavy bullets for rifles without enough twist that would also struggle with a accuracy.
I think there are some folks out there that think there aren't any drawbacks to going way beyond what is needed twist for a particular bullet. This is why I'm asking for what you guys have observed. And to add to this discussion, stated barrel twist rates are not what they actually measured in 4 barrels out of 8 that I've owned. 10 twist was measured at 10.75, 8 twist was measured at 7.5, another 8 twist measured at 8.25. Another stated 10 twist started out at 11 twist and gained to 10 at the muzzle ( cheaper buttoned barrel). All of these barrels mentioned weren't high end barrels. But I know of high end barrels that didn't measure as stated also.
I'm about to purchase a barrel for 300 Norma, I'm leaning towards a 9.5 twist but considering a 10 twist, 225 elds are what I want to shoot. Hornady states 10 twist to reach full stability. I also do not want finicky load development. From my thoughts & experience a 9.5 twist should cover what I want to do. I run the jbm stability calculator & varied ambient conditions, 10 twist should still keep me above a factor of 1.5. 9 twist, at times may put me over a factor of 2.

Your thoughts fellows?

 

[Canhunter35]

This subject interests me, lately I've been doing load development with a 11.25twist 308 and walking the edge of stability. I've achieved excellent accuracy on marginally stable loads, but it's such a pain I would ensure you have enough stability. And it's sucks to have an accurate load to 400 yards then have the bullets wobble past that and keyhole

Id get a barrel that can stabilize 2nd and 3rd choice of bullets in case the 225s don't work out.

 

[L.Sherm]

9.5

 

[DJ Fergus]

This subject interests me, lately I've been doing load development with a 11.25twist 308 and walking the edge of stability. I've achieved excellent accuracy on marginally stable loads, but it's such a pain I would ensure you have enough stability. And it's sucks to have an accurate load to 400 yards then have the bullets wobble past that and keyhole

Id get a barrel that can stabilize 2nd and 3rd choice of bullets in case the 225s don't work out.

What bullets were you running? I've shot Some 170 Berger's through my 10 twist 270 wsm. They grouped pretty good @300 yds and decent @ 500yds but groups were consistently Wilder @400yds but I think it has something to do with my bipod on the table I'm shooting from & angle of the bipod feet (slipping when fired). It wasn't keyholing.

 

[RockyMtnMT]

Ballistic stability and terminal performance are not always on the same page. Ballistic stability is heavily effected by altitude and temperature. A bullet that is calculated to have 1.5 sg at 4000' elev at 50* will most likely struggle at sea level and 20*. None of these conditions change the rpm's of the bullet. In our testing for terminal performance it became very apparent that higher stability aids in a bullets ability to perform properly after impact. Testing showed that the higher rpm's would keep the bullet point oriented longer after impact and ensure more consistent deformation and longer linear penetration. This particularly became an issue when a bullet that would calculate to be marginally stable at higher elevation and shooting accurately would suffer terminally. Say a bullet that is 1.4sg at 5000' elev but would be 1.2 sg at sea level, would act like a very marginally stable bullet upon impact.

For myself I prefer to hunt with a bullet that is over 1.5sg calculated at sea level regardless of the elevation that I still be hunting at. I think the closer to 2.0sg calculated at sea levelthe better your on game performance will be.

If hunting is not the end goal then ballistic stability calculated for current conditions is all that matters for getting on target.

 

[DJ Fergus]

Ballistic stability and terminal performance are not always on the same page. Ballistic stability is heavily effected by altitude and temperature. A bullet that is calculated to have 1.5 sg at 4000' elev at 50* will most likely struggle at sea level and 20*. None of these conditions change the rpm's of the bullet. In our testing for terminal performance it became very apparent that higher stability aids in a bullets ability to perform properly after impact. Testing showed that the higher rpm's would keep the bullet point oriented longer after impact and ensure more consistent deformation and longer linear penetration. This particularly became an issue when a bullet that would calculate to be marginally stable at higher elevation and shooting accurately would suffer terminally. Say a bullet that is 1.4sg at 5000' elev but would be 1.2 sg at sea level, would act like a very marginally stable bullet upon impact.

For myself I prefer to hunt with a bullet that is over 1.5sg calculated at sea level regardless of the elevation that I still be hunting at. I think the closer to 2.0sg calculated at sea levelthe better your on game performance will be.

If hunting is not the end goal then ballistic stability calculated for current conditions is all that matters for getting on target.

 

[DJ Fergus]

Good info, definitely helpful in making decisionsons on twist.
Thanks

 

[sable tireur]

For myself I prefer to hunt with a bullet that is over 1.5sg calculated at sea level regardless of the elevation that I still be hunting at. I think the closer to 2.0sg calculated at sea levelthe better your on game performance will be.

I think that this is the common sense approach since you start in the area of stability at the lower elevation and then increase your probability (sg) as you go higher in elevation. It's a no fail process.

The only concern is bullet construction. With the Hammer solids, you are concerned with impact velocity for performance. Cup and core bullets also but can be adversely affected by more twist than necessary, especially with thinner jackets, while stability is affected with less twist at any altitude, temperature or velocity.

This is one of my favorite reference works about bullets:

http://ffden-2.phys.uaf.edu/212fall2001_Web_projects/Isaac Rowland/Ballistics/Bulletflight/index.htm

It can be a little daunting at first glance but read a little and sleep on it. Then read some more. Pretty soon all sorts of lights start to come on.

Regards.

 

[Canhunter35]

What bullets were you running? I've shot Some 170 Berger's through my 10 twist 270 wsm. They grouped pretty good @300 yds and decent @ 500yds but groups were consistently Wilder @400yds but I think it has something to do with my bipod on the table I'm shooting from & angle of the bipod feet (slipping when fired). It wasn't keyholing.

208eldm shot well to 400 then fell apart before 500.
Now I'm testing 210long range bt and 195tmk. Shot ladders at 450 and they look promising, especially the TMKs.

 

[Rich Coyle]

Is a 9" twist fast for a .224 bullet? In my Savage .223 9" twist the worst group it ever fired for five shots was a .494".

 

[WildRose]

Ballistic stability and terminal performance are not always on the same page. Ballistic stability is heavily effected by altitude and temperature. A bullet that is calculated to have 1.5 sg at 4000' elev at 50* will most likely struggle at sea level and 20*. None of these conditions change the rpm's of the bullet. In our testing for terminal performance it became very apparent that higher stability aids in a bullets ability to perform properly after impact. Testing showed that the higher rpm's would keep the bullet point oriented longer after impact and ensure more consistent deformation and longer linear penetration. This particularly became an issue when a bullet that would calculate to be marginally stable at higher elevation and shooting accurately would suffer terminally. Say a bullet that is 1.4sg at 5000' elev but would be 1.2 sg at sea level, would act like a very marginally stable bullet upon impact.

For myself I prefer to hunt with a bullet that is over 1.5sg calculated at sea level regardless of the elevation that I still be hunting at. I think the closer to 2.0sg calculated at sea levelthe better your on game performance will be.

If hunting is not the end goal then ballistic stability calculated for current conditions is all that matters for getting on target.

Quoted just because there is a whole lot of sound logic here that needs to be repeated.

My advice would be to go with a 1:9 twist just to be safe and because in all likelihood you will find yourself wanting to shoot heavier higher BC bullets at some point that will be marginal at best in a slower twist.

I would never build a rifle planning it around just one bullet. What will you do if it doesn't shoot that one bullet well enough to suit you?

No sense limiting your options, it could end up being a very expensive mistake in the future.

 

[sable tireur]

Is a 9" twist fast for a .224 bullet? In my Savage .223 9" twist the worst group it ever fired for five shots was a .494".

It's not the weight or the diameter of the bullet which determines the twist rate. It's the length in combination with the other factors.

 

[Nieko]

I am going with a 9 twist on my next 300 Norma barrel. I have a 10 twist now and it will stabilize 215 bergers for target shooting but terminal performance is poor. Shoots 210 hunting vlds well with good terminal performance. My favorite bullet for the Norma is the 230 berger which actually stabilizes better than the 215 but I believe the terminal performance will be better with a 9 twist.

 

[Rich Coyle]

It's not the weight or the diameter of the bullet which determines the twist rate. It's the length in combination with the other factors.

I guess I should have mentioned the bullet was 52 grains.