Originally Posted by
Bass Ackward
I think this is the wrong approach and I am going to come at this from a different angle.
Let's reverse the thought process and define slow problems first.
Take a wadcutter out of a handgun. Poor BC. Even though RPM loss is minimal, a perfectly stabilized wadcutter destabilizes and becomes in accurate fairly quickly. Why? Can't be RPMs making it go wild. Must be air resistance. So the answer is to increase RPM with a faster twist rate to compensate for stabilization. Cause if you try to drive the wadcutter faster, it will lose accuracy at the same RPM that a faster twist would stabilize it at a lower velocity.
Take that wadcutter and put a nose on it. Now it's called a semi wadcutter. This improves BC and bullet balance by making it easier to stabilize with weight shifted to the rear of center. But not necessarily making it more accurate at close range distances. Just easier to drive faster. Load that with a low power charge and it too will lose stability at fairly short distance, probably at a longer range than the wadcutter. RPMs remain constant there too, over this short distance but air (wind) really causes the inaccuracy. Shoot 7 grains of Green Dot in a 44 Mag with a 250 Keith in several guns with the same exact twist rate and all will have the load go wild at different distances even though RPMs and velocity are fairly close to the same level.
Same as some 38 twist rifles that can launch 300 grain bullets well and others that can't at the same velocity level, the same (low) RPM level. Can't be too low or no guns would perform well. The typical velocity range for these handgun loads (above) that air affected them, are less than 1000 fps. This equates to 788 MPH. Now when accuracy is lost, these results are fairly understood. Over RPMs are not involved in the inaccuracy. The slugs became destabilized even though RPMs were constant causing air to have more effect. Increasing the velocity of poor BC slugs makes it more difficult to obtain accuracy when it is easier to simply spin it faster with a faster twist if stabilization is necessary.
All basically correct. However what you are discussing is the minimal amount of velocity required to maintain stability. Each design has it's own limitations at the bottom end. Nothing new there. What does this have to do with cast rifle bullets at the velocities we are discussing. Stabilization is not the issue.
But change to rifles .... and we lose our common sense when velocity really increases. Air goes from having a major effect on accuracy on a poor BC at low velocity to having ZERO effect what so ever when we double or tripple veliocity.
I have no idea where you get this idea?
Load and pressure means nothing to launch at these levels when we KNOW it does in really low levels of both RPMS and pressure. The fact that LArry shoots my bullets differently in his guns is the fault of RPMs and not the mechanical difference in the guns that we readily understand at low levels. Our ignorance cause us to look for excuses.
Again i'll refer you to the fact that I am comparing the proportional difference in accuracy with the same rifle. I am not comparing the accuracy of my rifle against yours. I also am looking for consistant results. Not something that can only be done under limited circumstance with one rifle. Also I'll remind you this is test is about regular designs of cast bullets, not custom made ones for a specific cartridge/barrel/throat. You might also remember I have already agreed that acceptable accuracy can be had above the RPM threshold. I've asked you many times to take a 311291 and shoot it in increments from 1700 fps to 2500 fps with a powder of your choosing in your own '06 with 10" twist and to show us the 5 shot groups. You've not done so because you know what the results will be. And those results will substantiate the RPM threshold.
Is there a sweet spot for for RPMS? Same question can be asked is there a sweet spot for velocity? And what happens if we increase wind resistance by increasing bullet diameter? We lower the ideal RPM or velocity level. Which means we lower the accurate velocity level. Even though 22s are the hardest to mold well, (balance) they have the highest velocity potential. An inaccurate 22 caliber (jacketed or cast) is most often more accurate than a 30 caliber at the same RPM level. Why? Has to be less wind resistance, cause it is just as highly a destabilized slug or worse.
The whole purpose for spinning a projectile is to allow stabilized flight through air to minimise the negative effects of air. Without air, RPM is a non factor. No bullet spin or over spin, good balance or bad balance, all slugs would fly true in a vacuum.
Quite untrue. The centrifugal force acts the same; in air or in a vacuum. How is it do you suppose a spinning space station "creates" gravity without air?
So air REALLY causes all inaccuracy, not RPMs. Assuming a bullet exits at the same point in the harmonic position of a barrel which handguns do not deal with. But handguns do see barrel time and recoil as similar effects to harmonics.
Could you run a test to prove this hypothysis or at least provide some scientific proof? I really think you're in left field here Bass.
Do anything to increase air resistance by a bullet at launch, whether that be poor balance, increased twist rate angle, or increased velocity and RPM, all increases air resistance.
Easiest accuracy occurs at the point that rotation is sufficient for the length of the slug to stabilize it at the lowest .... velocity .... possible that is required to reach the distance of interest. Anything above and beyond that point magnifies wind resistance and inaccuracy.
It is quite proven that given equal shapes (BCs) the fastest bullet is less effected by wind.
You can have just as lousy accuracy at low velocity / RPMs as high all because of air. If you can manage to launch a cast bullet true, 200,000 RPMs won't affect it. I need a bigger case to go faster to tell you where the top actually is. Right now I have a 625-2 that is RPMing me to death. But I will beat it. :grin: