Originally Posted by
Larry Gibson
Eutectic
Your 'proportional amount' is talking to you (us).... It may be in a language we don't understand at the moment. But it's a clue to success......
I think you are misunderstanding what I meant by "proportional amount(?). The "proportional amount" I refer to is simple a quantification of the difference in accuracy capability between the 3 rifles. Let's say at the best accuracy (intrinsic accuracy) one barrel produces with the best of match jacketed bullets is 1.3 moa accuracy, the 2nd barrel 1.6 moa and the 3rd barrel 1. 75 moa. Then with our cast bullet load the 1st barrel gives 1.95 moa, the 2nd barrel 2.4 moa and the third barrel gives 2.6 moa. We can say the accuracy is consistent because each load is about 1.5% of the intrinsic accuracy with each barrel when it is compared to it's own intrinsic accuracy. However, if the same load then produces 3.1 moa in the 1.3 moa barrel and 2.6 moa in the 1.75 moa barrel we know something is afoot as the "proportional amount" is now askew at 1.5% to 2.4% difference. Basically "proportional amount" is comparing the accuracy difference of each barrel to itself before we compare it between the barrels.
No, I understood what you meant Larry. That increased % of in-accuracy is what I said was talking to us.
While I understand your goal here; I think your use of the term 'RPM Threshold' is giving too much credit to a variable which has many variables already in its makeup. I certainly am not discrediting all your hard work on this! You are working hard for some repeatable baselines which is good. Please let me throw out some thoughts as this is not my first rodeo.
The title simply is a title. The RPM creates centrifugal force which is the real culprit. There are a lot of variables which is why the RPM threshold is not a hard and fast figure or "limit" as some want to think. The RPM Threshold is that point generally between 120- 140,000 RPM where, because of the many variables, that accuracy will begin to degrade.
"RPM".... This term is usually used for the rotational speed of an object that rotates in a stationary position. Our boolit has both rotational speed and linear (forward motion) speed. An engineer can calculate the RPM of a cast iron flywheel and the resulting surface feet per minute to know if said flywheel is safe at that RPM. Our boolit is adifferent story...
Let's take your 14" twist gun for our example. You know the muzzle velocity from your chronograph. So some simple calculations gives us an "RPM". IS THIS A REAL NUMBER?
Put another way..... when your boolit leaves the barrel is it in fact making ONE REVOLUTION IN 14"? How about 100 yards? I've tested a lot on these questions; so let me answer: One revolution in 14"? The boolit can make one revolution in 14" just out of the gun. BUT IT MIGHT NOT AS WELL! At 100 yards? NO! The distance traveled per revolution becomes LESS! It will continue to change and will not be 14" again! I've proven this and so has Mann over one hundred years ago. This is the definition of "going to sleep" by the way.
Would like to see how you've managed to "prove" that.
Larry, I take it you haven't read Franklin Mann's book?
I am not to keen about posting proof as it created quite a stir last time! If you remember some five years ago we had a rather heated thread on this 'proof'. Not you and I, but another that adamantly argued a boolit COULDN'T make one revolution downrange in a length less than the twist length of the gun it was shot in. He was wrong and didn't like it much.....
It is a ballistic fact that rotational speed of the bullet degrades very, very little over the normal muzzle to target distances (0 to 1000+ yards)
Why my above statement is true.... Sorry you misunderstood I didn't know this.
How do we "know" the rotation is 1 in 14" out of a 1 in 14" twist barrel? Law of physics says a body in motion tends to stay in motion unless acted upon by another force.
Like 'torsional distortion' maybe?
The bullets velocity decreases because it is acted upon by another force; the air density. The rotational force degrades very, very little (there are calculation formulas for this in most ballistic books btw) because there is little to no air resistance or another force to act upon a stabilized bullets rotational speed.
So out of a 1 in 14" twist barrel we can say for certainty the stabilized bullet is making 1 turn in 14" and thus correctly calculate the RPM on that.
Certainty may not be correct Larry. If the boolit loses its 'locked in' grip (Like widened land runners maybe) You may just lose you 1 in 14" with some certainty.....
Bottomline... Being RPM is related to distance in our case with our boolit; it is a variable in its own right and not necessarily accurate or trustworthy.
Bottom line that is an incorrect assumption. Would the bullets rotational speed slow down that quickly the bullet would then lose rotational stability at a very close range and we could not shoot the distances we do with accuracy and bullets would not have the maximum ranges they do. It is proven that the rotation sped does not slow down appreciably
Sorry Larry.... That was a poor way of wording my sentence The distance I spoke of was the 14" twist to determine probable RPM.
Let me throw out what may be causing the proportional amount discrepancy you are seeing... I think maybe 'torsional distortion' ... Almost ALWAYS for me when I see a serious accuracy degradation with fast twists at fast speeds; and if I can find or recover a fired boolit; I see WIDER land engravings on the boolit than actual land width! This 'torsional distortion' can really mess with distance traveled per revolution!
And what then would cause this "torsional distortion" to make the bullet inaccurate by not following the line of flight? Let me answer that; "torsional distortion" imbalances the bullet by moving alloy and thus moving the center of gravity away from the center of spin.
Not necessarily Larry... The 'torsional distortion' caused by high velocity, higher pressures, and steeper twists. Maybe all three... This distortion can displace metal evenly around the circumference of our boolit by widening land engraved width. You could still be in balance but you may not stay against the lands' driving edge... Humm?
And then pray tell what causes the center of gravity not coinciding with the center of form and the center of spin to lose accuracy? Centrifugal force is what. And then where does centrifugal force come from? The answer is RPM.
Your thoughts on more 'bearing' surface on the boolit is a good one Larry... I agree if HV is the goal. But if we think what is imparting our 'spin' to our boolit, the length is short. Even 5/8" is little to guide one turn in 14" as we leave the muzzle. With a perfect boolit/rifling fit you'll guide the rotation to 1 in 14"... If you wallow out the land travel you might get anything for 1 in ??" 'Torsion distortion' is aggravated as the twist goes faster for any given velocity that initiates it.
The % of bearing surface to bullet length is not important for the reason of "imparting spin" to the bullet.
It may be important to 'hold' our "imparted spin" as we push the envelope though???
It's importance comes from it keeping the bullet aligned concentrically from case to throat, in the throat through engraving on the lands and in supporting the entire bullet. The more of the bullet we have at groove diameter the less there is to bend, slough or obturate in any direction to imbalance the bullet. That long bore rider has a lot of leverage to bend, slough, or obturate it one side or the other into the .003 - .004 (usual groove depth). Also in the case of the Loverin designed bullet the lube in all those groove is not very compressible (laws of hydraulics) and supports the bearing surface as does a PP or jacket somewhat. You don't have that with the shorter bearing surface long nose bore riders. Even with a 50/50 land to groove ratio you still have 50% of the bore rider nose that is unsupported. None of the bearing surface is unsupported. Thus it is better to have more bearing surface and less nose.
I understand this point Larry. "Bore riders" do seem more correct for accuracy from a design
perspective... but I fight them more times than not!
I think I would add your same exact test boolits, but sized down and paper patched to the equation... I know you want to achieve a 'bare boolit' application... But this would be a good indicator if 'torsional distortion' was the culprit of your 'proportional amount' changes if they suddenly improved using a boolit with better traction!
Not necessarily so as the PP does a lot more than simply provide "a boolit with better traction". If you (I already have) tested a pure lead, a COWW and a linotype cast bullets PP'd and worked up loads from 1800 fps to 3000 fps in a 10" twist '06 you will find that even PP'd bullets have an "RPM Threshold". Actually jacketed bullets do to. The RPM Thresholds are just higher.
Granted, one can 'push' any of them hard enough to create centrifugal force problems outside the bore. My comment was for a suspected in the bore problem I call 'torsional distortion'.... maybe circumferential distortion would be more clear? At any rate, what you were shooting could be handled with PP even at 1 in 10" with ease as hard a boolit as you were using.
Your input is appreciated and has given us additional food for thought. You're actually pretty correct on a lot but are only looking at the obvious and not yet seeing the root causes. Look deeper with the laws of physics and ballistics in mind and you will see many of the "variables" we can change and affect. However, the laws of physics and ballistics we can not change but we can affect them by changing the variables.
Larry Gibson