Elevation which to load

[Ironworker]

I have a 6mm Creedmoor with 7.5 twist . It shoots the Sierra110 MK under 1/2moa and the 108 Berger also under 1/2 MOA at 5,000 ft elevation . This June I'll be out side of Billings MT for Prairie Dogs at about 3,000 Elevation . The box on Sierra bullets says 7"twist needed,Berger says 8" twist . Is it possible the Sierra shoot great at 5,000 FT but not so great at 3,000 FT . Wonder if I should shoot the Berger 108s in instead ?

 

[Ranger188]

I shot the 110g smk's at 750 ft. in a 7.5 tw. 6 Creed
Shot great.

 

[284winner]

You'll be fine with the 110 SMKs. At 3100 fps and at 3000' asl your bullet should stabilize easily.

 

[Idaho-Lefty]

I'm trying the 95 gr. Berger, Classic Hunters, in my New 6 XC, 1- 7 1/2 Twist at, approx. 3,100 FPS for Varmt's.
LESS recoil and they should, "Buck" the Wind good enough for, as far as, I can hit, a Rock Chuck, PD or, Sage Rat.
Have little doubt about, the accuracy of these bullets with, the proper, Tuning / Load, BUT,..
will they, EXPAND ??? I'll know, in a Month or, two !
Ans to, the OP, the 108 Berger, Elite hunters, "shoot" in the 3's and 4's in my 1-7.5 TW Criterion bbl. at 3,200 ft. Elv.
The 110 MK's, COULD BE, "Borderline, stable", in the 7.5 TW at, the lower elevations.

 

[284winner]

I'm trying the 95 gr. Berger, Classic Hunters, in my New 6 XC, 1- 7 1/2 Twist at, approx. 3,100 FPS for Varmt's.
LESS recoil and they should, "Buck" the Wind good enough for, as far as, I can hit, a Rock Chuck, PD or, Sage Rat.
Have little doubt about, the accuracy of these bullets with, the proper, Tuning / Load, BUT,..
will they, EXPAND ??? I'll know, in a Month or, two !
Ans to, the OP, the 108 Berger, Elite hunters, "shoot" in the 3's and 4's in my 1-7.5 TW Criterion bbl. at 3,200 ft. Elv.
The 110 MK's, COULD BE, "Borderline, stable", in the 7.5 TW at, the lower elevations.

Never really know till you shoot them. I shoot the Berger 95s in my 9 twist .243 at 800' asl with excellent results.

 

[Idaho-Lefty]

Windy and cold Yesterday but, was able to get, a 1/2 " group, on first try with, New Norma, un-fireformed Brass, old Win Primers ( from 1980's ) and 41.7 grs of, StaBall 65. with, the 95 Classic's,.. IT's, a "start" !
Noticed that, Bullets still had slight, Rifling marks, on the edge of the Bearing surface, WTF ?
I must have, mis-measured, the COAL, as I wanted about, .030 "Jump" into the .104 Freebore !
I will move Bullet IN about, .030- .035 more, and try Load again ! Wanting / hoping for, 1/4 to 3/8 ths, MOA.
But for now, I gotta wait for, better Spring weather, to re-test !

 

[Idaho-Lefty]

Iron,
To answer your question, on the LAST line,. I personally, would NOT take a chance on a possible, UN-stable Bullet / load, after driving that,.. FAR ! WHY chance, being disappointed ?
I would shoot, your 108's or, something that, weigh's / recoils, a bit less, IF you can find a load ( watch your Velocity on the FAST Twist with, lighter bullets, tho ! ).
The 87 grain V-Maxes, 80 gr. Sierra #1515 Blitz Varminters or, 95 gr. Berger Classic's, ALL shoot, pretty good
( sub 1/2 MOA when doing MY Part ) in my 1-7.5 twist, 6 XC.
Those Sierra 80 gr Varminters, with IMR 4350, are VERY accurate in MY Rifle, up to 3,200 FPS and they ARE, "available", too !
Good Luck !

 

[Ned Ludd]

The thing to remember about this topic is that the term "unstable" gets thrown around a lot with minimal qualification as to how it's really being used. The term "unstable" should be really defined as causing effects such as keyholing or oblong holes in the target. Bryan Litz used to recommend a minimum gyroscopic stability coefficient (Sg) of 1.40 as being sufficient to garner the full intrinsic BC of a given bullet. He later increased that recommendation to an Sg of 1.50. Some have interpreted achieving an Sg sufficient to obtain the full intrinsic BC of a bullet as "stability", when it's not necessarily the same thing in practical terms. Obviously, a twist rate necessary to achieve an Sg of 1.50 would be faster than that necessary to achieve an Sg of 1.40. However, running a given bullet with an Sg as low as 1.25 or 1.20 doesn't necessarily mean the bullet will fly erratically, potentially keyholing at the target. It generally does mean the bullet will behave as though it had a lower BC than the advertised value. However, someone running a bullet with an Sg of 1.25 may never actually observe oblong holes or keyholes at the target. So there is a gyroscopic stability coefficient window in which the bullet will still fly with sufficient stability to reach the target without having an erratic trajectory, but will likely have an effective BC slightly lower than expected. That does not mean it is "unstable". When the Sg gets down around the 1.10 range (or less), that is when major stability problems such as oblong holes or keyholing usually start to show up. Nonetheless, you can often run a barrel twist rate with a given bullet that generates an Sg in the 1.20 to 1.30 range, and the bullet will remain stable; however, it will behave as though the BC is lower than it really is.

As elevation increases, the twist rate necessary to stabilize a given bullet to the target will decrease slightly. Along the same line, less elevation travel will be required to zero at a given distance, again due atmospheric effects. The best thing to do is run a given bullet at the known elevation (altitude) and expected temperature, together with the known velocity and barrel twist rate, using a program such as Berger's Twist Rate Calculator (https://bergerbullets.com/twist-rate-calculator/). Although such programs may not be "perfect", they usually give a pretty good idea of the relative Sg values that will be generated for a given bullet/rifle setup in the given conditions. The good part is that it can be done entirely in silico, only requiring the necessary input values. One can play around with differing elevation, temperature, twist rates, etc., then look at the predicted effect on the Sg and BC (outputs). Even if a bullet is predicted to have an Sg of of 1.20 to 1.30 from a certain twist rate barrel under specific conditions, that does not mean it will necessarily be "unstable". Certainly values closer to 1.50 are better, but the bullet will likely still fly true, merely behaving as though its BC was slightly lower than expected. Thus, a Twist Rate Calculator is a useful tool to compare various possible scenarios and come up with a pretty good idea of whether a given bullet will really be unstable (i.e. having a predicted Sg of less than 1.10 to 1.20), or merely behave as though the BC was a little lower than expected.

 

[Ccrider]

The thing to remember about this topic is that the term "unstable" gets thrown around a lot with minimal qualification as to how it's really being used. The term "unstable" should be really defined as causing effects such as keyholing or oblong holes in the target. Bryan Litz used to recommend a minimum gyroscopic stability coefficient (Sg) of 1.40 as being sufficient to garner the full intrinsic BC of a given bullet. He later increased that recommendation to an Sg of 1.50. Some have interpreted achieving an Sg sufficient to obtain the full intrinsic BC of a bullet as "stability", when it's not necessarily the same thing in practical terms. Obviously, a twist rate necessary to achieve an Sg of 1.50 would be faster than that necessary to achieve an Sg of 1.40. However, running a given bullet with an Sg as low as 1.25 or 1.20 doesn't necessarily mean the bullet will fly erratically, potentially keyholing at the target. It generally does mean the bullet will behave as though it had a lower BC than the advertised value. However, someone running a bullet with an Sg of 1.25 may never actually observe oblong holes or keyholes at the target. So there is a gyroscopic stability coefficient window in which the bullet will still fly with sufficient stability to reach the target without having an erratic trajectory, but will likely have an effective BC slightly lower than expected. That does not mean it is "unstable". When the Sg gets down around the 1.10 range (or less), that is when major stability problems such as oblong holes or keyholing usually start to show up. Nonetheless, you can often run a barrel twist rate with a given bullet that generates an Sg in the 1.20 to 1.30 range, and the bullet will remain stable; however, it will behave as though the BC is lower than it really is.

As elevation increases, the twist rate necessary to stabilize a given bullet to the target will decrease slightly. Along the same line, less elevation travel will be required to zero at a given distance, again due atmospheric effects. The best thing to do is run a given bullet at the known elevation (altitude) and expected temperature, together with the known velocity and barrel twist rate, using a program such as Berger's Twist Rate Calculator (https://bergerbullets.com/twist-rate-calculator/). Although such programs may not be "perfect", they usually give a pretty good idea of the relative Sg values that will be generated for a given bullet/rifle setup in the given conditions. The good part is that it can be done entirely in silico, only requiring the necessary input values. One can play around with differing elevation, temperature, twist rates, etc., then look at the predicted effect on the Sg and BC (outputs). Even if a bullet is predicted to have an Sg of of 1.20 to 1.30 from a certain twist rate barrel under specific conditions, that does not mean it will necessarily be "unstable". Certainly values closer to 1.50 are better, but the bullet will likely still fly true, merely behaving as though its BC was slightly lower than expected. Thus, a Twist Rate Calculator is a useful tool to compare various possible scenarios and come up with a pretty good idea of whether a given bullet will really be unstable (i.e. having a predicted Sg of less than 1.10 to 1.20), or merely behave as though the BC was a little lower than expected.

Great explanation. Now, if I could just remember what you said.

 

[Idaho-Lefty]

Ironworker, probably wants to be able to hit, a Target, the size of, a King sized Coke bottle at, 500 - 600 ( or more ) yards with THIS Load and I'm NOT sure that, the "load" is up to, the "task" at, LOWER elevations !
There's another thread on, the Trials of "trying" to make, the 110 Sierra, "Shoot" !
Sadly, they DON'T "shoot" for, quite a few, 6mm shooters !
There's lots of "stories" about, the Pro's / Con's, on the 110 Sierra Bullets, Post, so, read up !
I think it needs, a 1-7 Twist to be SURE that, it "stabilizes" at, DISTANCE / some Elev's !
Personally, I wouldn't drive, Hundreds of Miles, to shoot PD's with, a "possibly", POOR, load !
Just my 2 cents

 

[Idaho-Lefty]

Here's MY "story" on, UN-stable Bullets.
Almost everyone on here ASSURED me that, 55 gr Nosler Ballistic Tips would "work" in, a 1-14 Twist, Rem .22-250 bbl. !
And they did, in the Summer's heat ( higher Velocities, when Powder is warm ).
I was getting, Dime sized groups in June, July, etc. BUT in the Winter at 20* - 30*, the load opened up to 50 cent sized groups, at 100 yards and Silver Dollar sized groups, at 200 yds,.. YIKES !
Unless, a Load is PROVEN at, distance, Elev. and weather, I'm NOT, driving very far, to shoot Varmints with, it !

 

[Idaho-Lefty]

The 95 gr. Berger Classic Hunters are shooting, under 1/2 MOA in my Tests ( still ongoing, W/ seating depth)
And they, SHRED Sage Rats !
Not the dramatic, Flip em' in the Air that, the 87 V- Maxes provide, but seems, to kill and EXPAND,.. "well" with large gaping, holes ! Don't let, the Tiny Hollow Point, fool you,.. they "work" !
They are, consistently accurate for me, in a 1-7.5 twist, Criterion bbl. at slightly below, Max velocities.
The faster I spun them, the bigger, the groups got and they seem to like, a bit of "Jump" ( .030 for my bbl. )
Remember,... you gotta, hit em',.. FIRST !

 

[David Milisock]

I have a 6mm Creedmoor with 7.5 twist . It shoots the Sierra110 MK under 1/2moa and the 108 Berger also under 1/2 MOA at 5,000 ft elevation . This June I'll be out side of Billings MT for Prairie Dogs at about 3,000 Elevation . The box on Sierra bullets says 7"twist needed,Berger says 8" twist . Is it possible the Sierra shoot great at 5,000 FT but not so great at 3,000 FT . Wonder if I should shoot the Berger 108s in instead ?

I believe you're over thinking this.

 

[Ackman]

I believe you're over thinking this.

Yes.

 

[Ackman]

Elevation affects ballistic coefficient, as does temperature.

 

[David Milisock]

Elevation affects ballistic coefficient, as does temperature.

Agreed however, are you only going to shoot at one elevation?
I'm a field shooter and I tend to work max or very near max loads for field performance. I test in high heat for safety.

Then I use my years of experience to place shots, in my experience eddy currents of the wind crossing the crops and land contours are for more effective at changing the trajectory then if you're shooting in calm conditions at 5,000 or 7,000 feet elevation.

Work a good load and practice.

 

[Ackman]

Agreed however, are you only going to shoot at one elevation?
I'm a field shooter and I tend to work max or very near max loads for field performance. I test in high heat for safety.

Then I use my years of experience to place shots, in my experience eddy currents of the wind crossing the crops and land contours are for more effective at changing the trajectory then if you're shooting in calm conditions at 5,000 or 7,000 feet elevation.

Work a good load and practice.

I hardly ever shoot paper, just varmints mostly at 6500' or so, and as low as 3500'.

 

[David Milisock]

I hardly ever shoot paper, just varmints mostly at 6500' or so, and as low as 3500'.

I prefer moving targets but it makes working loads tough!