Recognizing this is more pertinent to our sidebar, but it seems relevant to the OP’s consideration of “overspinning bullets.”
@South Prairie Jim - I’d contend you do know plenty about measuring and calculating bullet RPM. Either you’re playing dead here, which I expect since you’ve shot enough to know better, or you simply don’t realize the information you have in your hands already, however surprising it might be that you just haven’t ran across the idea of calculating spin rate for your bullets.
Anyone with a chronograph and a spec sheet or a cleaning rod can calculate the RPM their bullet leaves the barrel. Just need your muzzle velocity and your barrel twist.
RPM = bullet velocity in ft/sec * 12”/ft * 60sec/min / twist rate in inches/revolution
So a 3200ft/sec load in a 1:7” 6 creed: 3200*12*60/7 = ~329,143 RPM.
A reference for what should be common knowledge, or easily interpreted by folks understanding unit conversions and angular velocities:
Accurate Shooter article calculating bullet spin rate
A quick google search of “Varmint bullet max rpm” yields this “independent study,” with manufacturers referenced:
Shooter’s Forum Varmint bullet RPM reference**
**admittedly, I might suggest at least a common 20-25% design factor over some of those bullet ratings in the thread. As an example, I’ve fired thousands of 50 vmax’s at ~300-350krpm, while that link provides 290,00 as the limit.
You’re also welcome to conduct your own shooting experiments or contact manufacturers yourself to determine the designed ideal and maximum angular velocity for their respective bullets.
@taliv &
@<*(((>< - my statements above (and below) aren’t meant to imply bullet diameter (and mass) don’t contribute to centrifugal force acting to tear the bullet apart, but rather to state it appears to me that bullet design resisting the apparent centrifugal force is often a much bigger “knob” in terms of balancing spin rate against structural integrity.
Short Version: we shoot most bullets a long ways below the limit of their structural integrity. Most, but not all, such caliber matters, and speed matters, but we can and do make bullets with incredibly varied structural integrities.
Long version:
“Over spinning” a bullet will tear it apart when the centrifugal force overcomes the structural integrity of the bullet. It should go without saying for most of us, but that’s the basis here, so we’re talking about an equation like “if X > Y, bullet failure.”
Why do I believe design influences tolerance more than caliber influences force?... because we can easily calculate relative force ratios, and we observe some bullets tolerating far greater force than that which destroys others.
So let’s play first with the centrifugal force side - the force acting to tear bullets apart. F = m*w^2*r. Note: the mass here really can’t be considered as bullet mass, as a longer bullet in a given caliber with increased mass won’t have an increased centrifugal force acting within the bullet - but the proportional increase in mass relative to a caliber increase is fair. In a simplified view: Consider this like water pressure. A longer pool doesn’t have higher pressure at the bottom, but a deeper pool does. (Spoiler alert: it only gets worse and falsely makes my case appear even stronger if you mistakenly use bullet weight instead of radial density). Also of note here, since F = m*w^2*r, and the increase in reference mass m is proportionate to the increase in caliber, increase in r, we have a ratio solution relating caliber to force such Force varies proportionately with the squared caliber increase and the squared spin rate increase. In that light, spin and caliber have the same influence on force. Pretty significant when you think about it. But I digress...
If we compare a 22cal to a 6mm, for the sake of simplicity, spinning at the same angular velocity (like a 6 creed or a 223 at 3200 in a 1:7.5) - mass and radius increase by 9% good for a net increase of 18% force in the 6mm than the 22. Increasing caliber here increased the force 18%.
But what if the speed changes too? A buddy of mine on our state PRS club blew up a bunch of 75 Hornady BTHP’s at 2920fps last year in a 1:7”. That’s only 300,000 rpm. Alternatively, I don’t blow up 6mm 105 hybrids at 3200 in a 1:7” barrel - 330,000rpm. So 9% greater mass, 9% greater diameter, and 10% greater angular velocity (recall, this term is squared, so it contributes a 21% increase), that’s a 42% increase in apparent centrifugal force acting in the bullet.
If caliber is the “big knob,” why didn’t the 6mm 105 BTHP hold together against 42% increased force while the 22cal 75 BTHP blew apart?
Similarly, I’ve ran thousands of 50 Vmaxes at 3350-3400 in 1:7”, which yields ~350k rpm, and a 36% increase in centrifugal force acting inside the bullet. If caliber is the big knob, then why does the 50 Vmax survive 36% greater force than the 75 BTHP?
Why is a Blitzking rated for 350,000 rpm but a Varminter only rated for 216,000? That’s 2.6 times greater centrifugal force!
A 338 Lapua firing a 250 at 2900 in a 1:9.375” only spins ~220,000 rpm, but it’s greater diameter and mass mean it has ~27% greater centrifugal force acting than those destroyed 75 BTHP’s in 223.
For me, that’s all enough evidence to show that both sides of the equation matter, and show that the designed structural integrity influences the tolerance considerably more than the caliber influences the force. We aren’t running many bullets at or near their limits, so far away we might not even be able to reach it. In other bullet designs, we’re walking a ragged edge, and a slight increase destroys the bullet.
