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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/13/2008 at 11:14
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so i started playing with this today and in the original format 150*diamtersq/length i figured out that the constant(150) really didnt work so well, they mentioned using 180 instead which works better, anybody else use this? i see it can be quite helpful in determining the proper bullet weight for your rifle based on its twist.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/13/2008 at 11:26
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I have Greenhill's Formula set up in an Excel file and use it all the time.
When using this formula I have found that some manufacturers are rifling their barrels for the light for caliber screamer crowd. I sure don't want to get going on that one, now.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/13/2008 at 11:27
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is there any way you can send me that doug??
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/13/2008 at 11:48
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I'll email it, pronto.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/13/2008 at 15:41
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here is another although not greenhill will run directly in matlab
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/13/2008 at 15:44
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150's generally ok up to 2800fps.  use 180 over 2800.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/13/2008 at 16:14
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            The 150 up to 2800 worked for me when trying to figure out the twist to go with for a 9.3x62.  Couldn't decide if 1-12 or 1-14 was best. The Greenhill formula worked out to 1-13.something(can,t recall) so I decided on 1-12. Should work for all bullet weights.
 
til later
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/14/2008 at 19:55
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ive been running this on some of my bullets and the numbers arent adding up for some reason
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/14/2008 at 20:21
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I don't have my flash drive with me. Let me know what bullets and caliber you are running and I'll take screen shots for you. I'll then posst them here. It definitely works and is Greenhills to a T.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/14/2008 at 20:28
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what im saying is that according to the program i aint shooting the right weights in my rifle
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/14/2008 at 20:32
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The heavier the bullet the faster the twist, in general terms. Greenhill's proves this out.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/14/2008 at 20:44
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my twists arent agreeing with my bullet choices, but my accuracy is pretty good
here is the fun part when i use the 180*d^2/L it gets worse


Edited by pyro6999 - April/14/2008 at 20:51
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/14/2008 at 20:51
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This may be true, especially when using boat tails. The formula takes into account bullet length but not the center of gravity for the projectile. It is a generalization, but it does come reasonably close.
Play with some numbers for the 5.56 for example. Run the 55gr., 62gr., 69gr and 75 gr weights. I think you will see a correlation with the common twist rates in the AR-15 market. The heavy bullets need a 1/7", the 1/9" is a compromise.


Edited by tahqua - April/14/2008 at 21:31
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/14/2008 at 21:40
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ok i ran it again and my .280 is off as is my .264 win my 6.5 is pretty close my 22-250 is dead on with the 180*d^2/l
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