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johnkonneker
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davidlawrence
richardhart1
Jlk .
Not to be disruptive. But that will depend on your twist . I have a very standard cartridge with a twist too fast . I can show 30 cals in 06′ and 308 from 8.5 to 12 in everyday of the shelf guns . Granted in 1965 that Savage 110 LH may have been special ordered with the 8.5 for a moose slayer but a 308 with a 12 twist wouldn’t be unusual rather than the more typical 1-10 .timmalcolm
This is very true. As long as you stay below 144,000 RPM you can get away with anything. Once you cross that threshold, things get really difficult with a normal bullet (I would say impossible if you’re talking about 30 caliber and above, and you need to be able to shoot and hit past 100 yards).
To figure your RPM, multiply your measured muzzle velocity by 720 and then divide that answer by your twist rate.Actually, and more to the point of your question, yes it is possible to overstabilize a bullet.
However, I do not personally believe that is why groups open up past a certain RPM. I don’t really even believe it’s the RPM that’s to blame. It’s just doing what it does when it gets ahold of an unsettled object.
With jacketed bullets, the jacket provides a safe place for the lead inside to get totally situated before it leaves the barrel, and the bullets are nearly perfectly swaged these days, so overstabilization is practically non-existent (unless you consider a grey puff at the 50 yard line a function of “overstabilization”).Getting back to cast bullets, it is my opinion that the bullet gets engraved in a twisted fashion because the part of the bullet that encounters the rifling first moves faster than the part of the bullet that contacts later. The lead has enough torsional strength to right itself after it leaves the barrel and it is reasonable to assume that it does so in exactly the same way that it was loaded with stress in the first place (that is, the part of the bullet that was first engraved is the part of the bullet to leave the muzzle first etc etc etc) once the barrel no longer has the ability to hold the bullet in it’s contorted position, and acceleration force of inertia is rapidly decreasing, the bullet is allowed to relax in flight, and unlike a jacketed bullet, it does so and does a significant amount of moving in the process. I believe this is the last thing you want for an event free flight into a small group at high RPM. I believe the RPM threshold is just the point where a typical 30 caliber cast bullet is overcome by centrifugal force caused by being spun in excess of 144,000 RPM.
Now, if this is true, then a softer alloy should be able to be shot at higher RPM than the hard alloys that we typically use, because the bullet would take a set as it is accelerated through the barrel and theoretically emerge in a settled manner in much the same way that the core of a jacketed bullet does.
That was the original intent of the 30XCB project to demonstrate this strange phenomenon but the three gentlemen I gave the original rifles to were unwilling to demonstrate this as they agreed they could easily do.
The only method that was conclusively proven to achieve high velocity shooting was the low RPM method that Larry Gibson proposed.
I still think there may be a place where a softer bullet may settle down and behave, but as of yet, it has not been proven in any reliable manner whatsoever.So the easy answer is:
“Yes, if you are shooting over 144,000RPM, you are overstabilized with cast bullets”
I would say that 99.9% of your rifles and loads will demonstrate this exactly.georgepatton
Larry Gibson
“Over stabilization” of a bullet, jacketed or cast, is a misconception. Goodsteel’s explanation is correct but let me clarify; every bullet requires a certain level of RPM to stabile (fly point forward w/o or at least with very limited yaw and/or pitch. Once the bullet, jacketed or cast, is stabilized no further increase in RPM will increase stabilization. It’s the same as if a woman is pregnant no further amount of intercourse will increase the pregnancy. Pregnant is pregnant and a stabilized bullet is stabilized.
What does happen with increasing the RPM above the RPM required for stability of a bullet is the increased centrifugal force created. This increased centrifugal force will have a larger affect on the imbalances in the bullet creating a larger divergence from the flight path. That translates into larger group sizes.
Larry Gibson
Interesting, I am puzzled why my 1:8 twist AR is shooting the MP 60gr Lino bullets at 2300 fps accurately? Everything else I shoot has been staying under that threshold though and are not cast from Lino. Is it the harder alloy compensates for this theoretical limit?
I have found that larger calibers have a lower RPMTH than the 30 caliber rifles, so it is reasonable to conclude that smaller calibers generally have a higher RPMTH and this has been demonstrated effectively by several people. I think that a smaller (or less massive) bullet cast of a given alloy is just inherently stiffer and tougher whereas a larger (more massive) bullet of a given alloy is softer, more torsionally unstable, and much more prone to slump, thus lowering the RPMTH, but theoretically improving one’s chances of getting a softer bullet to take a set on it’s way down the barrel. In the future, I will be trying to make this phenomenon occur, but I expect it will take a lot of shooting to realize it. In the mean time, the RPMTH is the beaten path I can depend on.
I figure an RPMTH of 133,000 or less for 35+ caliber rifles and as much as 155,000 for the 22s is a reasonable guesstimate. Let your own rifle tell you where the RPMTH lies with your combination of loads etc.Thanks GS, I started looking at some of my various loads that I have found accurate and it is the only one exceeding the threshold range you mentioned but indeed is 22 cal and a harder alloy than I usually use. I felt I could go faster with it as well but now have to wonder. I look forward to your testing when/if you get around to it. I am too busy stopping when the load makes me smile.
lassehansen
Lets reopen this old discussion
I have the 513-887-LFN-AG3 mold that drops bullets about 870gr with my alloy. I shoot these out of a Blaser R8 in 510 Førland (Norwegian wildcat simular to 510 Whisper).
V0 is about 1080fps – subsonic.
Custom barrels in 50 caliber is almost impossible to buy in Norway, and the only twist rate available is 1:10. I think this bullet should have about 1:20 in twist – so my barrel has perhaps way to fast twist for this bullet?
My groups are 3-5 moa at 100 yards – it should be 2moa or less for hunting purposes…
Mike B.
While it is technically possible to over stabilize a bullet by getting it to spin too fast…
What you have possibly left out is the particular powder charge that you are using… in this instance to produce the ~ 1080 fps subsonic velocity… that you originally noted…
It may be that the particular powder that you are using is not the best option combination for that particular cast bullet… it has tended to happen before.
I have basically duplicated the NOE cast bullet design in my bullet design software package … (+ or – 1 Grain) & have added it into a 510 whisper in my load and ballistics package… & got some rather interesting information using a 1:10 twist rate… in my ballistics software package I was able basically simulate your stated velocity.
I was then basically able to in the 510 whisper data able to basically duplicate your velocity, (+ or – about 4 FPS) along with an almost 100% burn.
My next step is to go to the European cartridge standard webpage and down load the particular design dimensions for your stated cartridge…
The assumption of course is that that particular wildcat cartridge case is listed on the European standards website…
One can only hope…
That’s so I can make an educated estimate, as to whether we are dealing with apples & apples, or apples and oranges… in this discussion.
Interesting question…
Take Care, & Stay Safe…
davidlawrence
richardhart1
timmalcolm
georgepatton
Larry Gibson
lassehansen
Mike B.- You must be logged in to reply to this topic.