The Accurate Reloading Forums THE ACCURATE RELOADING.COM FORUMS Rifles Big Bores Twist RateGo | New | Find | Notify | Tools | Reply | |
one of us |
What would be the best twist rate for 750gr-800gr .50 bore-rider/boat-tail? "Only the dead have seen the end of war" - Plato | ||
|
| Moderator |
At what velocity? George  | |||
|
One of Us |
I'm sure you're talking about solids, but this TWIST calculator should help you out.  "They who would give up an essential Liberty for Temporary Security, deserves neither Liberty or Security." ---Benjamin Franklin "SIC SEMPER TYRANNUS" | |||
|
| one of us |
If you run the calculator it will say 15" twist, which is the standard .50 BMG twist and those bullets would be around 2700 fps from most rifles. A lot of folks use 10" twist and 12" twist on the .500 A2 with complete satisfaction for bullets of 750 grain weight at 2150 fps, as well as superb performance with 570 grain bullets at 2400 to 2500 fps at up-close ranges. I like the 10" twist on a .500 A2. So did Art Alphin. The .50 Peacekeeper is produced with a 10" twist barrel by SSK. McGowen offers 10", 12", 15", 24", and 32" twists in .510 caliber. Shank diameter may be 1.200", 1.250", 1.375", or 1.500", in Chrome Moly or Stainless Steel. Those slow twist ones must be for lever actions and round-ballers.  | |||
|
one of us |
A 750 grain is spinning at 2160 revolutions per second in a 1-15 twist, so a 1-12 twist would have similar stability if you shot the bullet at "only" 2160 feet per second. | |||
|
| one of us |
Thanks for the info guy's!  "Only the dead have seen the end of war" - Plato | |||
|
One of Us |
IMPORTANT NOTE:
What about monometal solids? These would require completely different twist rates. Yes or no? Visit GS Custom for the relevant required twist rates on solids. OWLS My Africa, with which I will never be able to live without! | |||
|
| one of us |
Agreed, like bronze for example. "Only the dead have seen the end of war" - Plato | |||
|
| One of Us |
A discussion of bullet construction having an impact upon the formula can be found at G.S. Custom Bullet's web site. Gerard notes that.... "Gyroscopic stability is what makes a bullet fly in a stable manner. If the variety of conditions governing gyroscopic stability culminate in a gyroscopic stability value of less than one, the bullet is unstable and will fly funny. No amount of lecturing the offending bullet will make it straighten up and fly right. Something has to be changed to bring the gyroscopic stability value to more than one. Gyroscopic stability is a rather complicated subject and one often hears of the Greenhill formula being used to calculate whether a bullet is stable or not. Using the Greenhill formula is better than nothing, like some makes of chronograph, but for more exact calculations and a true picture, one must turn to the work of one R.L.McCoy. Using the Greenhill formula results in an inconsistent effect on the gyroscopic stability of a bullet if sectional density is changed. The equation does not ask for the right information to accurately take into account varying material densities and forms. Using McCoy’s method requires inputting the specific gravity of the bullet material as well as a number of form variations that will affect gyroscopic stability. Specific gravity denotes how dense a material is. If specific gravity changes, density changes. This must therefore lead to a change in sectional density! Eureka! A connection! Now we must investigate more thoroughly by making some comparisons to prove the connection. Still using McCoy’s method, we find that two bullets, identical in form but made from different materials, have different gyroscopic stability values as well as different sectional density values. Changing the sectional density therefore changes the gyroscopic stability. The connection seems to stand. As expected, two identical bullets, made from the same material have identical gyroscopic stability and sectional density values. Not changing the sectional density, leads to no change in the gyroscopic stability and we have two out of two. Now we are cooking! Now we compare two bullets, made from the same material, with identical sectional density values but with different forms, one a semi-wadcutter and the other a spitser boat tail. Alas, they have very different gyroscopic stability values and disprove once again what seemed to be a possible use for sectional density. It is form, rate of twist, diameter and speed that are the big hitters when gyroscopic stability is calculated. As usual, sectional density just tagged along as a coincidental by product of the important stuff." Best, John | |||
|
| one of us |
Thanks! Ummm, you wouldn't happen to have an R.L. McCoy bullet calculator would you?  "Only the dead have seen the end of war" - Plato | |||
|
| One of Us |
| |||
|
one of us |
I shoot 800gr AAA Boreriders in my .50BMG FCSA heavy class competition gun. The twist is 1:15. Best group 4.5 inches at 1000 yrds.-Rob Never underestimate the power of stupid people in large numbers to do incredibly stupid things- AH (1941)- Harry Reid (aka Smeagle) 2012 Nothing Up my sleeves but never without a plan and never ever without a surprise! | |||
|
| one of us |
Click here to download the calculator. You will need the specific gravity of the bullet material for the bullets you calculate. Copper is 8.95. John, the section you quoted above is from the article on Sectional Density and, although technically correct, it is somewhat tongue in cheeck.  | |||
|
| One of Us |
Hi Gerard, I picked up on the tone. Best, John | |||
|
| Powered by Social Strata |
 | Please Wait. Your request is being processed... |
|
Visit our on-line store for AR Memorabilia
