Bullet stability

[Neil S.]

I understand about being snowed in. I have to load all my stuff in a sled and tow it to my shooting bench with a snowmobile! I am starting to wonder if I seated the BK's out really far if they would stay straight, but it might just be a waste of time to try. The load I shot this weekend was 24.8 grains 10x, OAL 2.30" and unfortunately I didn't chrono it. I had previously gotten similar results with 25.1 gr 10x OAL 2.33" which I believe gave me an average velocity of around 3730 fps. Good luck at making your bullets tumble .

-Neil

[Fred_C_Dobbs]

All bullets wobble a bit immediately after exiting the muzzle, some more than others. It's called coning. All other things being equal, from the same barrel, a longer bullet should experience more pronounced coning than a shorter one. According to my references, the 39-gr SBK is 0.733" long and the 40-gr V-Max is 0.749" so if that's the cause, the keyholing should be more pronounced with the Hornadys.

[Neil S.]

Fred_C_Dobbs- Very interesting read, and it confirms some of my theories. I will test some 40 V-Max this weekend as I suspect they will also have sever "coning" at closer ranges. I guess this is a "little" bullet barrel .

-Neil

[Fred_C_Dobbs]

Fred_C_Dobbs- Very interesting read, and it confirms some of my theories. I will test some 40 V-Max this weekend as I suspect they will also have sever "coning" at closer ranges. I guess this is a "little" bullet barrel .

-Neil

Neil, I don't think it's as bad as all that. I quote from Bryan Litz's Applied Ballistics For Long Range Shooting, Ch. 10, Bullet Stability, P. 152:

"...All bullets are launched with some amount of pitching/yawing motion, however slight. If the bullet has adequate stability (as it will in most cases) it will quickly dampen out these minor disturbances and fly point forward with no practical consequence to the bullet's flight performance (BC)..."

"...[A]s long as the bullet has adequate stability (Sg equal to or greater than 1.4), then the way in which the bullet goes to sleep does not affect its flight performance...."

Litz defines stability as "the ability of a projectile to maintain its point forward orientation in flight, and return to that orientation if disturbed." Provided your bullet does not again exhibit keyholing at some further range, we can safely conclude it is "adequately stabilized" and the keyholing at 20 yards is nothing to be alarmed about.

However, as calculated by the Miller stability formula, if your rifle's twist is 1:10 and your MV is 3800 fps, with that 39-gr SMK, your Sg is only 1.27. Again quoting Litz:

"...If you aim for an Sg of 1.0, and conditions exist that cause it to drop to 0.99, you have a problem. It would be like designing a bridge to hold only up to its maximum weight, but would crumble if one more pound is applied. The 0.4 safety margin in Sg is there to account for the nonstandard atmospheric conditions, imperfectly balanced bullets, and errors in the prediction of the Sg value itself. The Miller formula used to predict Sg is accurate, but it's not 100% accurate with all bullets. There could be up to +/-10% or more error in the prediction of Sg...."

He goes on to say that the minimum Sg of 1.4 is not written in stone, merely a suggestion. The military uses Sg 1.5 to 2.0 because of the extreme environments and battlefield conditions they might encounter. And benchrest shooters, who use very precise components assembled with great attention to detail might go as low as Sg 1.3.

You can drive that bullet to 4500 fps from a 1:10 and still won't get Sg 1.4. But your 1.27 is awfully close to the BR shooter's 1.3, so provided you take care with your loading and don't shoot at temperatures below -40°, I don't think you have a problem.

[Wrangler John]

The stability calculator http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi shows the following:

You want a Stability Factor S.F. of 1.5 or greater - higher is better.

39 grain, .733" long, .204 bullet at 3,500 fps in 1:12" twist has a Stability Factor (SF) of 0.858 - unstable.
at 3,400 fps S.F. = 0.850 - unstable. The slower it goes, the worse it is.
This bullet can't be shot fast enough in a 1:12" twist to become stable - it's a waste of time, even at 4,000 fps. Cleaning the barrel is a waste of time.

A 1:11" twist at 3,500 fps the 39 grain bullet is marginally stable with a S.F. of 1.022. Getting there, but no cigar.

A 1:10" twist at 3,500 fps results in a marginally stable 1.236 S.F.

A 1:9" twist at 3,500 fps = a stable 1.526 S.F.. There we are, finally stable.

I use a 1:8.5" twist for anything above 35 grains and the 39 grain bullet would result in a S.F. of 1.711 at 3,500 fps.

My test results show anything less than 1:9" will produce instability with heavier bullets, or longer lead free bullets. The 1:8.5" twist shoots Berger 50 grain HPBT bullets very well. This applies to the .224 and .243 bores as well. The jackets aren't rupturing, the bore isn't rough or fouled, the twist is just too slow. In fact the factory 1:12" twist is a complete disaster for shooting a wide range of bullet weights and lengths.

We can all prattle on about this or that theory but I found that it works to just plug the bullet length, diameter, weight, velocity and twist rate into the JBM calculator and go from there. It has never set me wrong. Even the little 26 grain Varmint Grenade shoots amazingly well in the 1:8.5" twist at 4,110 fps. Quarter inch all day, and only because I can't shoot better than that. Bottom line, the 39 grain BK's aren't for you, try something else, or order a prefit 1:9" or faster Savage prefit barrel and swap it in.

Oh, also don't worry about "dirty powder" Reloader 10x, it shoots superbly in both my .204 Ruger and .223 Remington. Its the closest thing to an ideal powder for these cases. I clean with Bore Tech's Eliminator, then Cu2 Copper Remover, then C4 Carbon Remover. A few patches is all it takes.

[jo191145]

John

I'll agree the 12 twist is not the best choice in a 20 cal.

The load I use in my 6PPC rates a 1.009 SF when shot on a computer..........Go figure

[jo191145]

Decided to see what my Sav 12 twist does at 15yds. 24.8gns 10X
Results were just what I expected.

Lousy pics but the edges look sharp and clear in person.
One of the 40 V-Max has an what appears to be an unusual tear in the paper.
Cardboard underneath the paper does'nt.
Make of it what you will.

If nothing else I learned this. I'm no good at shooting offhand, taking pics or downloading them

[sand rat]

Neil

Could you do me a favor?

Get a camera that will take some very close up pictures of the crown of your barrel. They need to be as sharp, in focus, as they can be. Not interested in looking at your barrel, just the hole coming out of it.

Post the pictures so we all can have a look see.

Sand Rat

[Joe O]

Neil,I think you have given the Sie 39s a good shake out.I would shoot the 40 V Max and berger 35s to see how they do,otherwise stay with the 32s.You might change powders,ie,Varget,IMR 8208 XBR,H4895,etc.I have good results with 27.3gr of 8208 with the 39gr Sie OAL 2.355. 3750fps 26" barrel.

[Neil S.]

jo191145- Thanks for posting your results. I guess it prooves that every barrel is indeed different.

sand rat- I might have access to a camera that could take those photos and I will try tomorrow. I will have a chance to shoot this weekend and might try a couple groups of my 32 V-Max reloads for fun. Again a damaged crown seems unlikely as this gun shoots the 32's just fine and I cant see any dings....

Thanks for all the help with understanding bullet stability everyone!

-Neil

[jo191145]

Neil

Odd, very odd.
Quess my first theory was off
I would take a close look at the crown.
Odd that they all seem to be flying tip up. Seems strange to me anyway.

I've noticed on my most recent Sav 204R barrels the crown/bore junction is not particularily sharp.
Almost looks like someone hits it with a buffing wheel.
Its never been an issue for me with flat based bullets. Does'nt look proffessional but they still shoot.

If I ever dig out from this snow I'm gonna try some 39's at 20yds just to see what happens.

Quess I should ask. Have you ever shot any other bullet at 20yds?

Seems I need to correct this post.
The 204 tube I used yesterday indeed has a sharp crown.
It was the 243W tube I was toying with last fall and the previous 204R tube that had "washed out" crowns.
Its been a long fall/winter with other things happening besides shooting. Brains had time to rust over. My bad.

It might not be a ding. Perhaps just a radius is enough to upset your bullets.
Boattails are more susceptiable to this. As the bullet leaves the bore escaping gases push on the boatail. If they're escaping from one side of the muzzle prematurely you can quess the result.
If theres any truth to this theory my WAG is you'd see a more noticable radius towards the top of the crown.
Still just quessing and conjecture. All internet gunsmithing is though.

I do know the pictures you posted are not normal coning, gyroscopic prossession or anything else. Its abnormal and indicative of a problem. Could it be twist? Yes. 12 is not optimum IMO. I've spoke to the engineers at Savage about this topic. I suggested a 11 or 10 twist as standard. They insisted in thier testing the 12 twist was most accurate. What bullet(s) they tested I'm not sure. I know you can't argue with an engineer thats convinced thier right:lol:

TC came out with a 10 twist shortly after, Go figure.

Every barrel is indeed different. I think that does have something to do with improper twist rate.
I still think what your seeing is a little more than a finicky twist. Could be wrong, its happened before.

[ryutzy]

I found this topic very interesting. I do shoot the 39 SBK in a 24" bull barrel 1:12 twist from my AR. The calculations are interesting, however the 39 SBK shoots ok from my gun. I was even fairly happy with my 500 yard groups. The 40 grain vmax does not shoot very well tho, as stability seems to be an issue, so apparantly my 39 SBK is "on the edge." By the way I'm shooting the 39 at 3750 fps. I will load the 32 SBK and try those just to see if there is a difference from the 39ers. I do however, plan to have a 1:10 twist barrel installed for my next barrel.

[Fred_C_Dobbs]

Is my face red. The 40-gr V-Max and the 39-gr SMK both have plastic ballistic tips. An OAL measurement that includes the plastic tip is meaningless for stability calculations because the ballistic tip's material has a substantially lower density than the "regular" bullet material. Both will actually have a higher (more stable) Miller number than what they calculate to. You can't even draw any valid conclusions concerning the relative stability of the two to each other unless by sheer chance the length of their polymer tips is the same.

[jo191145]

Fred

By no means am I an expert in ballistics. I pay little attention to such matters.
It shoots, good. It doesn't shoot, bad.
First time I've ever heard of discounting the polymer tips.
In essence all match grade bullets have a hollow core in the front. No lead,just copper.
Which means they are lighter than average density in the tip also.
Therefor if density/weight at the tip of the bullet is an issue all this would need to be factored in.
Anyone calculating the absence of lead in some bullets compared to others?
I know the JBM stability calculator does not ask for any such specifics.

Throw in ogive shape and you have a total new arena for weight distribution.
VLD's being very light in the nose. Huntin bullets very heavy.
Ogive plays a big part in BC. Never heard it be considered in twist rate/stability.

Do you have a formula in mind that requires such calculations for stability?

[Wrangler John]

I started my barrel twist quest when a Sako L461 Benchrest single shot in 6mm PPC USA (1:14" twist) wouldn't hit the entire target at 100 yards with the 62 grain Barnes Varmint Grenade. Finally, aiming about four feet to the right and left, a bullet struck the target - completely sideways. So I ordered a Shilen Select Match 1:10" twist in the 6mm PPC with a .262" neck for a Savage Precision target action. It shot the Barnes bullets better (wow a new slogan) but still not to my satisfaction. Tried throating the barrel out so the bullet would seat just above the shoulder donut to get a bit more powder capacity and raise velocity. Still didn't shoot up to the 70 grain Sierra Match lead core bullet or any of a number of 55 to 60 grain lead cores in the Sako. I figured that the twist was still too slow and the cartridge way too limited in powder capacity, so I ordered a 1:8" twist Brux barrel in 6mm chambered for the .243 WSSM to get the PPC shape and more capacity. That did it! The Brux barrel places 10 shots in a .247" to .360" group depending on my ability to read conditions.

Being stuck with lead free bullets here at home, my experience is that the Miller formula is correct. The general theory behind the PPC cartridge is that a bullet should just be stable enough to work, for whatever reason I forgot, maybe has something to do with spin drift and other obscure mumbo jumbo about jacket stress, yaw and pitch and phases of the moon. My friend in Oregon, like me a complete obsessive about such things, tells me of a shooter that works for Sinclair International, that experiments with fast twist barrels, and mentions the 1:5" twist. He wins matches, so who am I to argue?

No one knows why certain bullets won't shoot in a given barrel. I have yet to get anything lead free from Hornady to shoot as well as the Varmint Grenades or the Nosler BT lead free, no matter the twist. Hornady bullets generally are not as accurate as those from Barnes, Sierra, Nosler, or Berger, although I'm talking fractions of an inch. In fact the Barnes 62 grain 6mm V.G. bullets can vary in length from .974" to .982" in a single box, but still shoot bug holes. Measure them with a comparator and they are spot on, so maybe length variation isn't as important. Bullet length and ogive shape only seem to matter in a marginally stable bullet. Spin them faster and those factors are less significant. So, using various "fast" twist barrels, in .204 and .223 cases I have come to the conclusion that 1:9" or faster is ideal over the widest selection of weights, cores and shapes. It took me a lot of procrastinating to order that first quick twist barrel, a $400+ mistake is nothing to sneeze at.