Minimum Bullet RPM

I had an excellent conversation with Paul at Sierra, about minimum bullet RPM and bullet stability, we zeroed in on they 107gr HPBT, and he was telling me that the minimum bullet RPM, that he has sent down range and still maintain stability, is 216000 RPM he achieved that from a 6 BR that he sent at 2400 FPS from an 8 twist barrel, Then I asked him about slower twists and if it would be possible to stabilize the same bullet, I told him about my other chamberings and so he proceeded to tell me if I maintained that min RPM it would stabilize, so we did some math and this is what we came up with, the formula is muzzle velocity X 720 ÷ barrel twist, remember 216000 RPM is the magic number, I have came about these findings myself on my own, but it was nice to hear it from the bullet MFG, I have found the number is even lower closer to 200000 when you start to get past the 3500 FPS mark, but I will stick with 216000 for the sake of argument.

2400FPS for a 8 twist barrel 6BR check

3000FPS for a 10 twist 243 AI check

3600FPS for a 12 twist 6mm06AI check

4200FPS for a 14 twist 6mmMachIV there is load data available to achieve this.

Dean

Dean although I understand where this threads intended point is, I'm confused because every other post of your's I've seen has been about maximum's? Just never seen you as a minimalist!

Anyways, how does projectile weight play into this equation or calibre like .30cal for instance. I was pushing 175SMK's at right about 2700FPS from a 26" 10 twist barrel with a buddies reloads. By your equation I get 194400RPM's. Is that RPM limited to that one bullet or is it a broader spectrum. Are we not able to push the 175's fast enough from a 308 in order to stablize them or do I need a 9 Twist barrel to properly stabalize them?

Basically the math I'm seeing shows that with a 10 twist barrel you have to be pushing at least 3000FPS in order to stabalize a round.

Believe you me it is about maximum, It is about the widest range of bullets you can get away with, the broader the better, as far your 30 cal it has more bearing surface so the number will be lower, and also trying to tell folks that there are other twist that work, and that 8 twist is not they end all, I like the fact that I can shoot group of 55gr Noslers and 115 Dtac's and then switch to some 87gr Vmax's, without blinking or giving it any further though, the numbers I posted were like I said minimum FPS numbers you need to stabilize them, I am fully aware not everyone feels as comfortable running the pressures I do, but the numbers I posted are easily attainable while staying within SAMMI specs, and like I said those are minimun numbers, I run substantially higher than those posted.

Thanks Dean

Excellent info Dean!!! Thanks! Will keep this for my wildcat work.

I knew there was an RPM factor involved but I had no idea what the minimum was. I knew a fast twist with the hot 22's would vaporize some lighter bullets due to twist rates......but didn't know what the lower limits were.

At they other end of the spectrum, the number where bullets start to vaporize, the continence among most bullet MFG is 300000 RPM.

Dean

I have always heard the Greenhill formula is the "best" predictor of minimum twist. I know sectional density of the bullet must be accounted for. I got some of the Lost River Ballistics CNC bronze bullets years ago. It took a very high twist to make them fly. Never could achieve that stability.

Greenhill formula: http://kwk.us/twist.html.

Luck, Tim