I would agree Tom but what is this in regards to?

J.Louis

I tend to think so also but one still can't get around the length, weight twist relationship so I am not sure what Tom was trying to say? In all honesty I am kind of burnt out on this whole issue. I am not trying to prove a point just trying to provide a viable means on how look for a used mould that is the proper bullet for the specific twist one has on hand without wasting money. Buying a new mould from the likes of Brooks, Jones, Mos etc. surely would simplify it as they would just custom cut the mould to fit the need. But it sure would be interesting to know how they calculate the length and weight to fit the twist.

J.Louis 

Joeb,
No, I have not found a reference stating that the original GH formula ever had a correction. If you have such a reference, I'd be glad to look at it.

Weight is needed in because of bullet shape. A 10 caliber ogive will have a different weight/length ratio (I suppose you could cal it SG) than a round nose, secant or hollow point bullet. I have 32 cal cast sptizers with ogive's of 3, 4, 5, 6 and 8. CG effects the stability also. the farther back it is, the faster it has to spin to remain stable.

And it's my experience and understanding that temperature effects stability also. As well as the transonic range and barometric pressure. Though, I have never recorded pressures when recording info in my testing.

Frank

frnkeore wrote on Dec 9^th, 2011 at 1:44pm:

Greenhill used a s.g. of 10.9 rather than lead s.g. of ?11.15? to account for the jacket on the bullet.
As you say,  think that it isn't s.g. that greenhill meant, but the weight of the bullet compared to ?something.
Don Eagan's catalog shows that his designs varied from 73.4% to 84.4%, average 80.4% of the weight of a cylinder of the same diameter and length. 
The % goes down as the bullet gets pointy or # grease grooves increases. My research shows an average of 82% for SS rifle-Pope style-lotsa grease grooves flat point bullets.
I'm working on the Miller formula to give WEIGHT. If I can wade through the arithmetic, we'll have the Miller formula with direct calculation of Sg, Twist and Weight. 
Meanwhile, I'm working on modifying Greenhill for velocity, temperature and pressure.
joe b.

Jeff I am with you and I am still trying to figure out why everyone was trying to fix something that wasn't broke in the first place?

I wish I would have never brought up the Miller Twist Rate Rule at this point, but I will continue to use it for my own benefit and those around me. 

J.Louis

Quote:

Joe, I worked the Powley Bullet Stabilizer Calculator - hard when Don Miller mentioned  that the Ideal 375166 bullet had the best ratio (.765) in a 1:18 twist and I finally got the mold.  Dead Nuts on 1:18 with bullet facts & 1105 fps 
Here's the 1st range test ... (You need to Login or Register to view media files and links)

Then my hands got some of the new 2010 lot of KIK, with a Uberti HiWall - 10 rounds over the chrono then 40 rounds working back from the 500m Rams to the Chickens.  Was totally happy the bullet could and did put the steel targets down consistently with 1.5F @ 1086 fps and FFg @ 1119 fps
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When John sent me D Miller's calculator, the bullet variables were imputed for the 375166:
1086 fps - Sg = 1.77
1119 fps - Sg = 1.79

Conclusion:  Both calculators identified the bullet velocity that was optimum - which I shot with accuracy in a 1:18 twist that the Powley said was dead nuts on for the velocity ranges and Miller calculated high Sg's

I am totally satisfied that both calculators are excellent support tools for load workups, especially Don's that is specific to black powder

Quote:

If you want the Powley Calculator that calculates twist, go to the 1st page of this thread.  I posted the Calculator as an attachment.  It will calculate variable twist rates (too fast or slow or dead nuts) with the bullet details and velocity




 

This subject has shown allot of interest so I thought it would be appropriate to share the below link. Please don't try to slice and dice it or to argue its validity. Just try to sit back and thoroughly enjoy it. It is my hopes you get something positive out of it for your use both now and in the future. When it talks about jacket failure and bullet blow up try to picture this as the unsupported nose of a cast bullet being thrown / forced / slumping off center and no longer being central to the bore. There is allot in common with accurate cast bullets and jacketed bullets if one just fills in the blanks. 

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J.Louis