Greehill Formula

[Todd Birch]

In looking for something else, I ran across the Greenhill Formula for calculating what twist will stabilize a given bullet weight:

"Divide the length of the bullet by it's diameter in hundreds of an inch.

Then divide the number 150 by the results of the first step.

Multiply the result of step two by the diameter of the bullet. The resulting figure is the necessary rate of twist for that particular bullet."

For example, using a .40 calibre bullet 1.30 inches long, dividing the latter number by the former produces 3.25.

When 150 is divided by 3.25, the result is 46.15. Now multiplying 46.15 by the exact diameter of the bullet (.409 inch) gives the final result of 18.87 inches, the minimum rate of twist necessary to stabilize a 1.30 long, .409 inch diameter bullet.
Can any of you .40 shooters verify that?

Another handy British method of determining the ideal bullet weight for a particular bore size is to multiply the bore diameter in inches by 5.5.

For example:

A .308 bore multiplied by 5.5 gives a figure of 168 (actually 169.4) , an ideal long range bullet weight for your .308, .30-'06 or .300 magnum.
A .277 bore calculates to 152 grs, a known performer in the famous .270 Winchester.

I think the firm of Holland & Holland came up with that one around the turn of the century.

I gotta find more to do......

Todd

[Lead Pot]

Well Todd I'm going to have to take your word for all those figures.I just dont have enough toes and fingers to carry the decimals far enough.
Man if you need something to do come on down and help me cast some bullets

Kurt

[Todd Birch]

Hey Kurt

You don't gotta work those numbers out in your head.
That's why God invented calculators and smart assed kids that know how to use them.

Actually, I blew it in my opening line....the formula is to calculate the best weight of twist for a known BORE size, not WEIGHT.

The other formula is for bullet weight.

Help you cast bullets? Don't hold yer breath waitin' Cowboy!

Those fumes rot yer brain.

Todd

[Lee Stone]

Now Todd, you know as well as I that lead does not volitalize (is that a world?) untill it reaches at least 1100 degrees F. So you CAN come down and help Kurt cast bullets and it isn't going to rot your brain.

[Todd Birch]

Too late Lee. It done happened already.

I'm at the point where I think it's fun to get belted by a gun damn near as long as me and would rather shoot than, than, well, lets just say it's the most fun you can have with your boots on......
Todd

"Life is not measured by the breaths we take, but by the moments that take our breath away."

Todd

[IronSight]

Todd,
Did you ever work for the IRS designing income tax forms?

[Lead Pot]

Naw Lead dont do nothing to you,heck I made it to retirement after....after..after Lee how many years did I plumb

Kurt

[Todd Birch]

Hey Kurt

I'm a retired gas fitter and natural gas worker.

For years we used a wonderful thread sealant called 'Masters Metallic Compound". It had lead in it. It worked.

When the "Safety Nazis" found out that it had lead, it was out the door. We replaced it with a series of non-lead compounds that left leaks all over the place!

Us old timers squirrelled away as much of it as we could find. Like Lee said, the only time it's potentially harmful is if you eat it or inhale it as we sometimes did welding near a fitting coated with the stuff.

Wearing gloves kept most it off'n your hide.
At any rate, aside from my passion for old rifles and obsolete calibres, I'm relatively normal.

Todd

"Faster cars, younger women, older whisky". Oh, and older rifles, too!

[RMulhern]

The 150 constant figure is ONLY....good for projectiles at 1800 fps and above. For more realistic figures for projectiles at 1100-1400 fps....use a constant of 120!

[RichBratlee]

Hey Todd , For some reason according to the math here--the British method works out just like you show for the .308 and .277---but when I plug in the numbers for my 45-70 it tells me a 251.9 grain bullet is ideal?? Or is this just another example of me having more to learn--- --again!!

Here's what I did--.458*5.5 and got 2.519--which works out the same for the 308 and 277 just have to move the decimal point

What'd I do???

Rich

[Omaha Poke]

Rich, that formula only works on velocities in excess of 1800 fps. That is probably why you are having trouble. Randy

[Todd Birch]

Hey Rich

Perhaps I ought to have pointed out that Holland & Holland came out with that ballistic gem at the time they were developing their .300 H&H "Stalking Rifle".

You betcha it was puttin' them out there in excess of 1800 fps! Same for the Canadian .280 Ross, great gran' pappy of the 7mm Magnum.

Make Greenhill work for you - go for1800 fps! Got a will made out?

Todd

[RichBratlee]

Ok that make sense guys!! I thought something had to be wrong!!! although--251 gr bullet--110gr 2f--hhhmmm might get that 1800fps!!! And Todd--NO!--Got a buddy shoots a 45-120 and I think he is trying for that tho!!! It hurts just to watch him shoot sometimes!!

[Clarence]

I can tell you that a 1-18" twist .40 caliber barrel will stabilize bullets in the 420 grain/1.3" length range. I've had two .40-70's with early Shiloh barrels, one on a Shiloh and another on a Rolling Block, and they both produce accurate targets out to 500 meters. I have heard that some people have had problems with bullets going wild just before the rams, but that a little more velocity stabilizes them for that far, so stability may be marginal. My newer .40's have 1-16" twists.

Clarence

[Todd Birch]

Hey Clarence

Any idea what velocity you were getting from those .40-70 loads?

There has to be a point where it ceases to work for projectiles under 1800 fps, or maybe it's a function of bore size.

We got any amateur ballisticians out there in Sharps land?

Todd