Monday, June 27, 2016

Bullet RPM

While it was scorching hot today, I was following a discussion over on one of the Gunslinger forums about bullet flight, and I started recalling some of the research that Junior Doughty and I did, back during our Castbullet days, and I began recalling the interesting things we found about bullet flight.  We noticed that over-driven cast bullets would sometimes come apart in flight, and we attributed that to a too-soft alloy being pushed too fast, with the resultant RPMs tearing the bullets apart in flight.

If you dig around on the internet, it won't be long before you find yourself at Accurate Shooter, a great resource for all things accuracy, and eventually you'll find that there are handy math pages, for things like calculating bullet RPM and how that affects bullet stability.

You can go to the link above, but the quick version of computing bullet RPM is:
RPM = MV X 720/Twist Rate
Where MV is muzzle velocity and you know the twist rate of your barrel.

For example, if we have a 12 twist barrel and the bullet is going 3000 fps, the RPM is 180,000 rpm. (3000 * 720 / 12 = 180,000).  I know of few things that can spin at 180K rpm and hold together, but bullets are one of them. Modern Jacketed bullets, anyway.  Some lead alloys start to come apart at considerably lesser speeds.  Junior and I settled on 125,000 rpm as about the top rotation that we could reliably put the old Lyman #2 alloy to without it coming apart.  The old Lyman #2 alloy was a standard alloy for many shooters, and it worked really well in the .30-30 Winchester cartridge when cast into the inestimable Lyman 311041, a bullet I really like.  Pushing it to 2000 fps in a 12 twist barrel gave us a bullet RPM of 120,000, which it would stand nicely.  However, if you pushed that same load down a 10-twist barrel (Like Marlin's standard .30-30 barrel) you would spin that bullet to 144,000 RPM and it would likely tear itself apart before it got to the target.  So, for a 10-twist barrel you'd have to keep the MV down around 1700 fps to keep the RPM under that magical 125,000.

That work was a lot of fun, and I'm glad we did it.  And, I'm writing this down here an now so that I can find it later without having to resort to my written notes, which are apt to be mislaid.  As I recall, I could launch my linotype alloy to about 1750 fps from my Marlin barrel without it coming apart, and there's noting wrong with a 170 grain bullet going 1750 fps in the piney woods of north Louisiana.  Many a deer, or hog, has succumbed to such a bullet over the past century, and it's cheap shooting.  Just about the most expensive component of that load was the gas-check on the base of the bullet.

Nowadays I shoot a lot more wax bullets than anything, but that discussion I referenced above got me to thinking about bullet RPM.  If we assume a muzzle velocity of 620 fps, and we use Ruger's 1:16 twist barrel as standard, that little wax bullet is spinning at 27,900 RPM as it travels across that 21 foot range. it will rotate 15.75 times before it hits the target.

That's something to think about, and I wonder, in my current game, how many misses can be attributed to bad bullets?  I know that inclusions and voids can affect the accuracy of old-fashioned cast bullet, and that the modern bullet makers today (Hornady, Sierra, Speer, Nosler, et al) take extreme pains to make their bullets as concentric as possible.

It's something to ponder.


Old NFO said...

Hmmm, damn good question, and one I'd never have though of. I'd 'assumed' the wax bullets were pressed into shape.

thinkingman said...

NOW... If you really want an additional exercise, calculate the Time of flight of the bullet to target, and factor in the velocity drop experienced in that distance to target ( without getting into instantaneous velocity calculations )and state how many revolutions the bullet experiences in that distance. More fun for the mathematically inclined.