[quote author=495D41444A405D4A2F0 link=1322775324/29#29 date=1323198493]I don't shoot beyond 200 yards with single shots so, if the bullet tumbles at 201 yards, it really doesn't matter to me. I know nothing about LRBP shooting. 

The Greenhill formula doesn't take temperature or bullet weight ( SD ) into account, nor velocity (it's not a variable in the formula) in any real terms.

I've used Greenhill since the early 80's, first in round balls and then when I got into single shots in '85.

At 200 yards, Greenhill over stabilizes bullets! If a bullet is rotating faster than it takes to stabilize it, the centrifugal force is acting on any imbalance within the bullet and magnifying it above what needs to be. I first started on this path with Ron Long in '85 when I bought my first bullet mold from him. He called it "sleeping", when the bullet settled down to it minimum stability.

Here are real world results and independently, they correlate with the DM program that I use from JBM. That program uses 1.3 as there lower end for stability.

Greenhill min. bullet length for a 16 twist .321 groove barrel is .960.
Correct
My Douglas 16 twist stabilizes a 1.12 lg bullet down to 1400 fps.
Greenhill says that it takes a 13 twist for a 1.165 lg bullet, my 15 twist Douglas stabilizes a 1.165 lg bullet. It will also stabilizes a 1.23 long 323471 bullet at 2157 fps. 
I get 13", .321" 1.189" bullet max, 15", .321, 1.030" bullet max. Greenhill


Greenhill says it takes a 12 twist to stabilize a .308 bullet that's 1.165 long. My Douglas 14 twist barrel stabilizes a 1.165 lg bullet and will stabilize a 1.21 at 2060 fps.

I get 14", .308, 1.016"max bullet length, Greenhill

Again, these results are at 200 yards and my results where independent from any stability program (just my experiments) but, they correlate with the DM program at 1.3 or above.



Frank[/quo

John;
Put on your algebra hat, look at the formula in the worksheet you sent, and you'll see.
joe b.



JLouis wrote on Dec 6^th, 2011 at 1:52pm:

Joe here is an example: If I change the bullet length to 1.2 and the bullet weight to 215 keeping everything else the same your program tells me that I need a 12.94522606 twist. The Miller program tells me I need a 14 twist with an  SG of 1.56 and as long as the SG falls between 1.4 & 2.0 the gyroscopic stability is where it needs to be. To use a 12.94522606 or to make it simple a 13 twist the SG is 1.81 of which is still fine but it might require a special barrel to be made with that twist so it over complicates things just a tad. Your program dictates the twist per say and with Millers you just type in the twist rate for the rifle on hand and you change the bullet length and weight to find the proper bullet for that particular twist and it makes life allot simpler. 

J.Louis 

Biggi and I have been messing around with the DM spread sheet.  She has Excel.   
We can put in all sorts of different numbers to stay within SG 1.4 and SG 2.  So how does one come up with the correct bullet length? All the bullets I have been using for almost thirty years in SS rifles fall within the 1.4 and 2 SG. I somehow managed this without an online twist calculator. 


          Joe. 

   

Joe,
It's more for a guy that has a rifle and needs a mold that will work (like what started this thread) or for someone that has a bullet mold and wants to know what rifle that it will work in. Or for someone that wants to try something new and wants to make sure that the bullet won't tumble before it gets to the target. 

There are lots of 16 twist 32/40's out there that won't stabilize the more modern spitzer, BSed bullets. In that case, you can at least know what length bullet to look for or how to mix and match what molds they already have.

If your bullet is tipping, (like on a 100 deg+ day) you can speed it up enough to straighten it up.

It's just a tool but, I think it's a good one.

Gotta find a way to stuff that lap top in my tool box and find that cell tower 

Frank

Excellent explanation of the usefulness of Miller's Twist Rate Rule! 

Thank You For Sharing It Frank.
J.Louis

Joe,
This is the Don Miller program that I play with. You don't need Excel for it.

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Frank

How slow is your twist rate and how long is your bullet?

Frank