Quote: Joe,
Such as density having to do with stability.
I will argue the point about Greenhill being satisfactory for the conditions you state. That's the very point why people a 100 yrs. ago, and today, feel it doesn't really suit our needs, and why many other formulas have been put forth in trying to find something that really works. Using the 150 constant just doesn't cut it in most cases. As a result some of us use constants of 125or 130 as we feel this gives a better result.
I don't agree with the premise that stability is a function of bullet length. Stability is given to a bullet by the rotation of it. So it doesn't make any difference the length as long as the twist is correct for it.
Pete et al;
First, bullet stability is precisely defined mathematically, I just don't know how to do the math. This topic has been defined for years.
Bullet Density. Greenhill assumed a bullet specific gravity of 10.9. a pretty hard alloy or a bullet with a copper or ? jacket. Lead has a s.g. of 11.34, 20:1 lead:tin has a s.g. of 11.1X.
Then with soft = more dense bullets the required twist is less, so I'm thinking the 150 could be increased slightly.
Stability is a function of bullet length for any given twist. Means that at the edge, longer bullets are less, and shorter bullets are more, stable.
Above 2800 fps or so folks start to use 180 in greenhill, they've lost a little density but the velocity more than makes up for it.
When you say that the 150 doesn't cut it, I disagree. My experience is that Greenhill adequately gives the MINIMUM twist required for any caliber/bullet length pair. In general, Greenhill probably gives us faster twists than we need with dense cast bullets at lower velocities. I just picked up a 45-450 Ohaus bullet and measured it, it's 1.094" long. At 125, twist = 23.9", at 150, twist = 28.6". My 45's have twists from 18" to 22". I know of no case where the predicted twist did not stabilize the bullet, and would be very interested in hearing of some.
When you mention using 125 or 130 in Greenhill, you are suggestng that twist should/must be increased by about 1/6 th. That's quite a bit. What is the rest of the story, gun, bullet, caliber, ctg etc?
I read about this transonic business, but am very skeptical. Most bullets go through the sound barrier at attainable = not 27 miles ranges. 22 rf at short ranges, others at higher ranges. 32/40's probably at 200+ yards? I've shot my 32/35 Maynard at 600 yards with reasonable accuracy, could keep them in the black. I doubt that anything horrible happens as the bullet goes through the sound barrier. Good shooters at long range shot great scores with 30/06's that most probably went under sos before the target. I've yet to see any evidence. Clearly the BPCR folks work with the dreaded transsonic region all the time. Probably just more opinion and theory masquerading as fact.
Again, I'd like to hear about when /what conditions greenhillo must have the 150 replaced by a smaller number, the twist must be faster.
joe b.