Sierra Heavy TMK 6MM Testing

[Jordan Smith]

Those twist rate calculators are best-estimates off very basic formulas. They're great to get you in the ballpark or let you know if you might have a problem. I definitely wouldn't take it as gospel and build a rifle/bullet system based off the calculator saying your SG is 1.05 and you're good-to-go and "moderately stable" with just lowering BC.

There are plenty of variables not captured in those basic formulas that affect stability (center of gravity, center of pressure, moment of inertia) as well as environmental factors like a stiff cross wind that can push a "moderately stable" bullet over to "unstable". I'm personally less concerned with the BC reduction, but the increased dispersion and erratic flight of "unstable" bullets are the deal-breaker for me.

If I have am shooting a rifle/bullet with a calculator output under 1.3, I'm definitely going to investigate stability with fired data.

Agreed on the sentiment that the 108 class bullets are the safer bet for an 8 twist at zero or negative density altitudes.

Absolutely. I would call the model a coarse approximation, not a "best-estimate", but yeah, I agree with you that, while it has practical utility, it's not a perfect model. It's a great tool for planning purposes, however, even though your actual results may vary when you get close to the theoretical extremes. If the theoretical stability of the bullet is under an SG value of 1.3, I don't even bother testing for instability. Instead, I stick to bullets with an SG value of 1.5 or greater to make sure I'm not working with a BC value that depends on atmospheric conditions. In reality, I build the rifle to achieve an SG value of >1.5 with the bullet I want to shoot in the most dense air I plan to shoot in.

Of course unstable bullets are the biggest issue, but my point was that even if the bullet is "moderately stable" according to the calculator, that doesn't mean you check the "stable" box on your list, and move on. The degree of stability matters and affects BC.

 

[nm.otter]

There are plenty of variables not captured in those basic formulas that affect stability (center of gravity, center of pressure, moment of inertia) as well as environmental factors like a stiff cross wind that can push a "moderately stable" bullet over to "unstable".

Hence, 4DOF and AB CDMs.

 

[mudslinger021485]

I believe @Ryan Avery said Sierra told him you really need a 1:7tw to get the full BC on with the 116TMK.

 

[Ryan Avery]

I believe @Ryan Avery said Sierra told him you really need a 1:7tw to get the full BC on with the 116TMK.

They don't say "need". They said that if you had a choice, get a 7-twist, but a 7.5 will work just fine for most.

 

[Jordan Smith]

Hence, 4DOF and AB CDMs.

Well, not exactly. Those apps likely use the exact same model for stability. They add additional degrees of freedom to more precisely model trajectory than some other apps, but stability is generally calculated the same way among the various different calculators.

 

[Jordan Smith]

They don't say "need". They said that if you had a choice, get a 7-twist, but a 7.5 will work just fine for most.

Mainly because it maximizes BC across the various different scenarios, as where 7.5T may not in some situations.

 

[nm.otter]

Well, not exactly. Those apps likely use the exact same model for stability. They add additional degrees of freedom to more precisely model trajectory than some other apps, but stability is generally calculated the same way among the various different calculators.

Interesting, and not my particular area of expertise.

That said, if I'm already doing (relatively) expensive 4/5/6 DOF math, why not derive SG directly instead of applying a simplified formula to previously derived results?

 

[Treelineasassin]

I’m sure it’s been mentioned somewhere in the 85 pages on this thread but in hopes of not having to scroll through all of them…

I just got a 6 creed 20” 1:8 twist. I wanted to run the 108eldm, but the 107 and 116 tmk have my attention after the latest exo podcast with Form. I’ll be going anywhere from 9,000+ elevation down to sea level on Kodiak and I’m on the fence if my 1:8 twist will stabilize the 116 at the lower elevations effectively?

I want to go with the 116 if it’s possible for an across the board bullet in my setup. Does anyone want to point me in the direction of one bullet in particular? Any experience with this same setup at these elevations?

Thank you!

Just run 107tmk, they're 100% stable in an 8tw. Doubt you'll see much terminal difference.

 

[Jordan Smith]

Interesting, and not my particular area of expertise.

That said, if I'm already doing (relatively) expensive 4/5/6 DOF math, why not derive SG directly instead of applying a simplified formula to previously derived results?

After looking a bit deeper, Hornady's 4DOF does appear to derive stability from it's high-dimensional model, but it's the only one I'm aware of that does that. Other than Hornady's 4DOF model, most calculators compute SG based on the Miller formula which is an adaptation of the Greenhill formula.

Why don't others use 4/5/6 DOF models to calculate stability? Because those 4/5/6 degrees of freedom are independent variables (6 degrees of freedom include all three translational dimensions plus all three rotational dimensions) used in trajectory calculations, which are non-trivially based on a set of coupled non-linear differential equations. Stability is a different sub-problem with a different set of dependencies emerging from simplified conditions, which can be derived from a higher-degree model like 4DOF, but something like the AB CDM doesn't model the motional degrees of freedom to do that, AFAIK, and instead uses traditional models like the Miller formula to calculate SG.

 

[Castle Rock]

They don't say "need". They said that if you had a choice, get a 7-twist, but a 7.5 will work just fine for most.

Yeah, there is a bit of ass covering there, but the obvious choice is a 7 in something like an ARC, and 7.5 in a magnum

 

[Ryan Avery]

Eh, I don't know about ass-covering. It says 7.5T on the box, and 99% of people won't have an issue with 7.5T, but it gives us something to argue about on forums.

 

[nm.otter]

After looking a bit deeper, Hornady's 4DOF does appear to derive stability from it's high-dimensional model, but it's the only one I'm aware of that does that.

I use it regularly for that purpose.

the AB CDM doesn't model the motional degrees of freedom to do that, AFAIK, and instead uses traditional models like the Miller formula to calculate SG.

I recall mention of some calculator using 6DOF -- in the past 12 months. Thought that might have been Litz, but apparently not. Thank you.

 

[Jordan Smith]

I use it regularly for that purpose.


I recall mention of some calculator using 6DOF -- in the past 12 months. Thought that might have been Litz, but apparently not. Thank you.

To my knowledge, AB is using a point-mass trajectory model with empirical drag curve data for the bullets, not a true 6DOF model.

EDIT: Maybe you're thinking of Lapua?

 

[Jordan Smith]

Here you go: https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

And for bullet lengths: https://www.jbmballistics.com/ballistics/lengths/lengths.shtml

An SG value below 1 is unstable, and over 1.5 is super-stable. Between 1 and 1.5 is moderately stable. Why should you care about how stable the bullet is, instead of just whether it is stable or not? Because the BC value depends on the degree of stability. For anything less than an SG value of 1.5, the bullet's BC decreases at a rate of approximately 3% per 0.1 in SG value. So a bullet with an SG value of 1.5 may have the advertised BC value, but with an SG value of 1.0 the bullet sees an effective decrease in BC value of ~15%.

The decrease in BC itself is not the biggest issue, but the variability in the BC as a function of atmospheric conditions can be a bit of a problem, depending on how precise you need your shooting solution to be (shooting out to 400 yards isn't far enough to resolve these issues, generally).

With a 6mm barrel and 1:8" twist, I default to the 108 gr ELD-M class over bullets instead of the 115 DTAC/116 TMK class of bullets, due to the fact that while the BC value may be lower, at least it is constant across atmospheric conditions.

Since we're talking about 6DOF versus point-mass trajectory models, and the Miller formula, it's worth noting here that the relationship mentioned above between BC and SG was not derived from first principles via a 6DOF model, but was determined empirically through experimentation done by Litz and reported in one of his books (Modern Advancements in Long-range Shooting: V1, IIRC).