Low Stability Symptoms - Question for Bryan

[swhiteh3]

I recently loaded up some .223 ammo for some stability testing in a few rifles, both 1:9 twist. The bullet in this ammo is 77gr Nosler CC, and I'm very aware that this twist is going to have marginal stability with a bullet of that weight / length.

The rifle I did some testing with usually shoots about 1/2 MOA at 100yds, and keeps very good sub-MOA groups out past 500 yds with 69gr SMK factory ammo. But with the 77gr ammo, it showed a massive vertical spread, nearly 3.5 MOA at 500yds (with 1 MOA to 2 MOA horizontal spreading). Could this be a symptom of under-stability? Groups at 100yds looked decent, but not great - just starting to show vertical stringing.

The 500yd targets showed no significant key-holing, or any other issues I would typically think of when I think of under-stability. So could this be a stability issue, and does it sound like a typical stability symptom, or am I fighting something else??

I wanted to investigate what stability does downrange, thinking maybe the stability was acceptable at shorter ranges, but was getting worse downrange, so I checked out the Berger Stability Calculator.

I entered in the information for my 6.5cm, and it gave a stability of 1.50. As I decreased the muzzle velocity, the stability factor got worse (less stable). But of course, changing the muzzle velocity also changes the rotation velocity as it's leaving the barrel, since twist rate is really a ratio of velocity to rotational velocity. So by changing the numbers in that calculator, we're changing our initial velocity AND our initial rotational speed - not a very good simulation of what happens downrange.

So does linear velocity decay faster than the rotation velocity? My engineering background says it probably does. But i don't understand the relationships involved enough to know whether the stability coefficient would get better or worse (up to the Transonic point, of course). Does the stability get better or worse as the bullet travels downrange?

 

[dcali]

You are correct that rotation decays much less rapidly than velocity. But there are two conditions to stability - gyroscopic and dynamic. Gyroscopic stability always increases down range. Dynamic stability sometimes goes wildly bad near mach 1, and is largely dependent on the bullet design. But you're no where near the transonic region at 500 yards, so I would look elsewhere.

 

[Bryan Litz Ballistics]

I think it very well could be marginal stability causing you to have a lower and erratic BC. Check out the most recent blog article in the Daily Bulletin here:

http://bulletin.accurateshooter.com/2015/03/how-ballistic-coefficent-varies-with-twist-rate-stabilization/

Sort of a coincidence you asked this question on the same day I sent this in for publication

Take care,
-Bryan

 

[swhiteh3]

Thanks Bryan! Frankly, I was shocked running the 140 Berger in a 1:8 twist barrel was as marginal as it was. Most people had said a 1:8 would be sufficient (and to be fair, in most conditions, it is), but clearly my next barrel is going to be 1:7.5 or 1:7.

In the meantime, I'm going through all my cold-weather data, and trying to "curve fit" the modified G7s from your Stability calculator to my data, and see how well it makes the fit.

As for my .223, do you think that vertical stringing could by symptom of low stability? Because in that rifle, it strings badly with 77gr but shoots 69gr very well.

-Scott Whitehead

 

[Berger.Fan222]

Maybe I am missing something. I can't see how a BC decrease of 10% or so can cause vertical stringing of 3.5 MOA at 500 yards.

Running the numbers in a ballistic calculator, a BC decrease of 10% would cause the group center to drop about 5.7" (1.1-1.2 MOA) at 500 yards, but why would the groups get bigger?

It seems to me that larger shot to shot variations in muzzle velocity or BC are needed to cause vertical stringing that big.

Assuming muzzle velocity variations are small, a 20% shot to shot variation in BC (0.193 to 0.151) would cause about 9.1" (1.8 MOA) vertical stringing.

 

[dcali]

That's another reason I said I would look elsewhere. It could very well be a combination of effects - bc variation due to marginal stability, poor velocity consistency, maybe inconsistent position, or even a rifle issue. My favorite (and probably the most common explanation for weird effects) is just wonky statistics. Sometimes weird things happen over a few shots that don't continue when you keep shooting.

You'd need some pretty wild variations for it to be ONLY the stability. Bryan probably has some info on how bad that can get.

 

[XTR]

Statistics is a neat thing. I deal with them in sample populations and radionuclide all the time. I found this interesting:


On another site I frequent a guy who is a statistician by trade challenged people to flip a coin 50 times, or not and make up answers, and record the results, 1=heads, 2=tails, in a post on the thread and he would look at the numbers and tell you whether or not you really did it or made up the numbers.

After a while posting this stuff he gave away one easy test he did. If you didn't have at least one run of 6 in a row it was a faked response. Apparently it's a really common occurrence if you run an a/b type random distribution for 50 events, but nobody making up numbers will put that many in a row.

I don't think he missed a single test.

What I've seen in environmental sampling that get evaluated with a Wilcox Rank Sum or a Student T test we typically need in the neighborhood of 17 samples in a set to guarantee that the results pass the statistical tests.

 

[swhiteh3]

Maybe I am missing something. I can't see how a BC decrease of 10% or so can cause vertical stringing of 3.5 MOA at 500 yards.

Me either, and that's why I asked the question. I was wondering if guys with far more experience than I have would say "Oh yeah, that's exactly what we see with low stability", but clearly that is NOT the case.

Since the rifle shoots other ammo so well, I don't suspect the rifle / shooter, so I have to take a close look at that ammo.

Thank you for all the replies.