How to determine max boolit length for barrel twist

[jdrslyr75]

I know about Greenhill's formula but it seems to me that this formula leaves many variables unaccounted for. I want to design some moulds that will cast a heavy for caliber bullet but I would like to know what my max length for each of my rifles is and work back to design a mould that will cast about a 95% max length boolit. Any advice from you veteran casters is greatly appreciated.

[357shooter]

Check out http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

Somewhere I had another link that had a good writeup about the JBM calculator. That test writeup claimed that stability was excellent all the way up to a stability reading of 5. That's different than what the web site suggests, but does seem to be true.

[jdrslyr75]

Thanks for the link I will check that out. Most of the info I have found online relates bullet weight to stability but they use j bullets for reference. Lead being denser than copper and other gilding metals makes it heavier so I am trying to find lead specific info and preferably length of projectile as opposed to weight.

Thanks,
Jeremy

[357shooter]

All of the ones I've seen rely primarily on length, the older Greenhill certainly does. Typically a longer bullet weighs more than a shorter one however, bullet profile can effect it too. There is no difference in the calculations for cast vs jacketed when using these calculators, at least the several I've looked at and used.

[Artful]

Stability will depend upon the twist rate of your barrel was well as the length of the
bullet and the velocity you sail it out at and velocity it impacts at plus the stability designed into the bullet.

http://kwk.us/twist.html

http://www.cbal.eu/stab.php

http://www.uslink.com/~tom1/twistrate.htm

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
Stability value should be between 1.3 and 2.0 to ensure stabilty (the military uses 1.5).

[BAGTIC]

Bullet length, not bullet weight, is the main factor though with two identical bullets with different weight due to different materials the lighter version will be less stable. The denser material puts more of the weight at a distance from the axis so that it produces a greater 'flywheel' effect. If a 'J' bullet is stable a lead version will be even more stable.