My shooting friend is working with a 1/8 AI 243. Working with virgin brass (not formed yet) he was loading 48 grns-H4831 below the 6mm 87 VMAX. He worked up to this load with no obvious pressure signs. After setting out toward the lands closer, he loaded quite a few of the same. The following day, the bullets were blowing up down range. For you who might not have seen that, it looks like a little puff of smoke 25 yards out or so.
We hadn't crony'd yet, and when we saw the smoked bullets, he wrote Hornady tech email. They said that 3300 fps developed 300,000 rpm and would take the Vmax apart. I concur with the rpm, can't crony cause I think I might smoke the chrony with an exploding Vmax.
Has anyone else seen this at this speed? I'm shooting from 1/8 standard varmint 243 barrel @ 3100 with no problems. 1/8 is twist.
I have seen that a lot of times, and the bigger the dia of the bullet the more it will have a tendency to do that. Just imagine the centrifugal force there is on those thin jackets at 300,000 Rpm. That and some bores a rougher than others, so it will do that at an even lower speed.
Dean
RUMs are like woman in Stiletto heals, you know they are going to put you in the poor house, but that has never stopped anyone from pursuing them.
I'll buy that. It was odd that Hornady tech didn't suggest FPS. Ditto to rough barrels. We shoot PD's and are always looking for longer range "solutions". Not much data out there for the 87 Vmax. I have just balanced between 85/90 load data tempered with a little 6mm rem data and am using IMR 4895 x 36.5 for 3100 fps. Sitting them out to lands, touching not jammed. I have had no problems. Guess we'll try loading down the AI by 10% and see if that blowing up goes away.
Had that happen years ago with 5.56/223's loaded with Hornady SX 55gr bullets and a 1:9 twist. They were fine in a 1:12 and even in a 1:10 at 3000 fps, but the 1:9 would show a grey 'poof' about 25-35 meters away from the muzzle, in about 10% of the loads.
Went to a 1:10 twist and moved to 2900 fps and the 'problem' disappeared. It was a problem, but just because I was using the SX bullets, which have very thin jackets and spinning them very very fast!
Reducing the load by 10% is a little drastic, I would go down 1 gr at a time I am sure you are just on the threshold of them coming apart and staying together.
Dean
RUMs are like woman in Stiletto heals, you know they are going to put you in the poor house, but that has never stopped anyone from pursuing them.
id load 1 round each at .3g lower to about 1.5g under your load now and shoot them from highest to lowest until they stop coming apart...in a 6.5 .5g equals about 20-30fps with 4831 so im sure its pretty close with the 243.
IIRC, the formula for bullet RPM is: Velocity X 720 divided twist= RPM
Example: 3300 X 720 = 2,304,000 / 8 = 288,000 RPM
This formula is not exact compared to what you get if you do the calculations long hand, but the difference is very small, about 3 percent. For example an 8 twist make 1 1/2 turns per foot of barrel travel. So 3300 fps is 4950 Revs per second, or 297,000 rpm, closer to what the tech said, and still beyond the rating of Hornaday loads. As I recall, Sierra rates match bullets to 330,000 rpm, but I might be off on that figure. I'd appreciate anyone who wants to correct me on that, feel free to chime in with a link or reference.
One gr made the difference between blow up and not. 47 vs 48 stopped the damage. Thanks for all the input.
"Classic Pressure Signs" merely tell you what the yield strength of the brass is, not if you are over SAAMI pressure.
Texas10 has your math correct.
I did a post a few years ago about this, I called all the major mfgr's and got limits on bullets. Anything less than about 5,500 fps is irrelevant to a bullet, what destroys them is RPM.
MV * 720 / Twist = RPM
Hornady V-max bullets limit is about 290,000 rpm, HOWEVER: Your barrel smoothness will play a huge roll in the absolute number.
I'm a firm believer in the theory that if it bleeds, I can kill it.
I called it, Do I win anything.
Dean
RUMs are like woman in Stiletto heals, you know they are going to put you in the poor house, but that has never stopped anyone from pursuing them.
You get the "booby" prize.
"An armed society is a polite society"
"...shall not be infringed" What's the confusion?
How about a shredded bullet jacket, if they can find the pieces. We found gliding metal fragments in the gravel where we shot... Small and very mangled pieces!Originally Posted by scope eye
I shot 87gr VMaxs as a prairie dog load in my 243 wssm Shilen 1/8twist with no problems. Velocity was right at 3350 and 3330 with Win 760 and Reloader 22. I never had them blow apart. I did vaporize 55 gr Midway Dogtown bullets in my 243 Striker at about 3100fps. It looked like a vapor trail to about 80 yards then nothing.
Should have cleaned your barrel first. Then scoped the throat. That is more than likely where the problem is at.
This "bullet blowing up" issue brings to mind a question I hope the OP doesn't mind my asking; Is there a predictable amount of bullet spin rate that, below fragmentation, the bullet is also not over stabilized?
For instance, if a shooter is experiencing a bullet spinning apart at say 288,000 rpm, and s/he drops velocity enough to reduce spin to 275,000 rpm successfully avoiding fragmentation, should the shooter also drop spin rate an additional say, 2-3% to avoid bullet over-stabilization and increase accuracy?
Is there a predictable relationship between the two? Or are there too many variables such as bore roughness, chamber alignment (center of mass misaligned to center of rotation) etc that also effect a bullets propensity to spin apart?
Sorry if this question is too technical this early in the morning.
I have talked to many bullet MFG over the years, and they have all told me the sweet spot for bullet rpm is around 250.000 rpm. And that is why I always order my barrels with a slower twist, I know how fast I am going to send them and choose the twist accordingly. it is always a slower twist than the usual main stream twist rate, also the slower twist allows me to send them faster. It is sorta like a catch 22 but in reverse. But hey it has always worked for me.
Dean
RUMs are like woman in Stiletto heals, you know they are going to put you in the poor house, but that has never stopped anyone from pursuing them.
^ Mr. Dean, I am glad to hear you say this, because I bought a 308Win barrel, throated for 190's, 11 twist. I haven't shoot it yet, but I was concerned it may not stabilize? I want to shoot heavy pills at 300yds or less, and they will be flat base, so as short as possible. Your post gives me encouragement that the 11 twist may not be too slow. Thanks...Jim :-) ^
For 30 cal and up the number is much lower than the 250.000, due to the mass of the bullet keeping the momentum. "flywheel effect"
Dean
PS: For those 190s I would stuff as much ball powder as I can in those cases IE 748, CFE223,
RUMs are like woman in Stiletto heals, you know they are going to put you in the poor house, but that has never stopped anyone from pursuing them.
Im pushing 190 smks over 3300 in a 10 twist with a 300 Norma look alike wildcat.
Maybe i should say the Norma looks like mine which came first. Anyway i dont think
theres much need for concern in blowing those bullets up in a 308. Im not aware
of any doing that in my gun, at least before they hit the intended target.
I think he was more worried about not being able to stabilize them, than blowing them apart.
Dean
RUMs are like woman in Stiletto heals, you know they are going to put you in the poor house, but that has never stopped anyone from pursuing them.
Yes, stability was my concern. I have 748 and CFE223. Should get to try it out next week or so.
Thanks...Jim :-)
Texas 10,
There really isn't an answer to your question, other than "it depends", but here is a little discussion about it.
There isn't such a thing as "over-stabilization", there can be an induced wobble if you spin super-fast, but below destruction RPM; IF the bullet ins't perfectly concentric. But there is no way to know if the ballance of the bullet would be X, nor if they would all have that same amount. So you really can't compensate for it, nor would any but the most advanced tests be able to pick it out when shooting.
As for "The sweet spot" for RPM, it is better to not think of hitting a number like 250,000 RPM, rather a spin ratio as can be found on the JBM site. The reason why slower twists work well, and what many don't think about is how everything is relative. A bullet needs a stabilizing spin based upon it's length, AND forward velocity. The spin is important to the velocity, thus the better description of a stability ratio.
As a bullet travels, the velocity decay happens relatively quickly. The bullets rotational decay happens VERY slowly. So as a bullet travels a great distance, it's stability factor actually increases; because relative to it's velocity, the RPM has increased.
I'm a firm believer in the theory that if it bleeds, I can kill it.
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