| eventually the velocity/RPM/stability threshold will meet and accuracy will go south.|
Do you have a formula for this?
This article basically pertains to jacketed bullets, I'm sure you have heard on occasion where someone having bullets shot from an AR with fast twist barrels blew up the bullet before they barely made it no more than past the muzzle.
As to cast lead bullets it's a whole different animal. This post by Larry Gibson who has done extensive test covers the main point. The subject has been argued back and forth on many forums but it's been my experience that it's more true than not. It not impossible to shoot cast lead bullets at factory jacketed bullet velocity but it requires using the right combination of bullet design, alloy hardness an good casting, proper powder and correct bore twist so that the forces imparted on the bullet by the velocity / twist rate ie RPM's doesn't accentuate the defects of the cast bullet and cause instability and tear it apart due to the weaker structure of the bullet.
| The RPM threshold is area of velocity where the RPM created by the twist accentuates the defects in cast bullets creating excessive yaw, wobble and pitch of the bullet during flight. Defects in cast bullets are caused by imbalance due to out of round casting, other casting defects, or unwanted obturation during acceleration. Exactly where that threshold is depends on numerous things but those that mostly affect the threshold are; burning rate of powder, quality of cast bullet, design of cast bullet, alloy, fit of bullet to throat/leade and twist of the rifling. I've found over the years that with normal commercial moulds generally available (what most of us use), casting quality bullets of WWs or harder, with proper lube and fit to the throat when using medium to slow burning powders the threshold is in the 125-140,000 RPM range. If we are using the old '06 with a 10" twist and 311291 then the threshold translates to somewhere between 1735 fps and 1944 fps we will find the best accuracy when using 4895 or a slower burning powder. |
With a faster powder such as 2400 best accuracy will come at a little slower velocity as the faster powder has a faster acceleration. This means unwanted obturation will occur at lower velocity so accuracy will deteriorate quicker than when the medium/slower powders are used.
|The cast bullet bench rest boys really slow twists down which does help considerable. If one is building a special purpose cast bullet rifle then twist is of major importance. My 14" twist Palma barrel (27.5" long) in .308 Winchester shoots regular cast bullets (30-15--FN, 311291, 311041, 311299) with excellent accuracy in the 2400+ fps which is 125,000 RPM, right at the beginning of the threshold. Really, really best accuracy comes down in the 21-2200 fps range which is around the 110,000 RPM area Junior1942 talks about. Same bullets in my M70 target with 12" twist 26" barrel loose accuracy at the same RPM as the 14" twist barrel. My M788 with a 10" twist 22" barrel does exactly the same. With the 12" twist that is in the 21-2200 fps range and with the 10" twist it is in the 1740-1940 fps range.|
I've found that most regular heavy for caliber cast bullets shot out of regular rifles shoot best in the 125-140,000 RPM range with medium to slow powders. One merely must know the twist of the rifle to figure what the velocity range for best accuracy will be.
Basically I have several know conclusions. With my current molds, alloys an casting techniques and powders putting together the best loads I can with the equipment I have I can pretty much match the above results in my 1:12 twist 30-30 an my 300 Sav. and all my 1:10 twist 30 cal rifles like my Mosin and SKS rifles.
Good article here as well goodsteelforum.com/for...-accuracy/