Bruce,
I would still advise in favor of the heavier bullet if wind drift is your biggest concern. For a given level of pressure, the heavier bullets may start out slower, but in the long run, they will have less wind drift at long range. Of course it's also important to consider the grouping potential of the bullets. I'm only talking about the ballistics.
Choosing the slowest twist that's adequate for stability is generally a good idea, but it's got little to do with BC. For properly stabilized bullets, the normal levels of precession/nutation that occur are so small (less than 0.1 degrees) that they don't affect BC a measurable amount. The same can be said of # and configuration of riflings. By choosing the slowest twist that stabilizes a bullet, you're minimizing the dispersion factors that are related to spin rate like bullet imbalance and in-bore yaw. In other words, not 'overspinning' the bullets can potentially help with grouping ability, but has nothing to do with BC. The exception to this statement is if the twist is too slow and the bullets are not adequately stabilized. BC can certainly suffer than.
Doug,
I'd like to discuss your observations of the 162 Amax fired in field conditions.
First of all, I have no interest in disputing your observations. I simply wish to illuminate some of the sensitivities that are involved in this kind of analysis (BC analysis).
When high BC bullets like the 162 Amax are fired at the high speeds you're talking about (3200fps+), the drop is quite insensitive to BC. In other words, you might have a relatively big difference in BC and it won't necessarily result in big differences in drop. For example, when I run the JBM program out to 1000 yards using the advertised BC (.625) and compare it to my measured BC (.580), both with 3200 fps muzzle velocity, there is only 9" difference in drop at 1000 yards.
I believe you when you say that you get accurate tables when you use BC=.625. But would you really notice if your tables were off by 9" at 1000 yards?
Here's another example. A common practice is to enter the measured muzzle velocity into the program as the actual muzzle velocity. This introduces error into the calculation due to the bullet slowing down between the muzzle and the chronograph. If you entered 3200 fps as the muzzle velocity, and your muzzle velocity was really 3210, and you used .625 for the BC, now there's only 7" error in your tables. Would you notice that?
You mentioned that you prepared the tables ahead of time and carried them out into the field. This makes it impossible to guarantee that your drop table is perfectly suited for the atmospheric conditions of that shot. If you prepared your drop tables assuming some set of atmospheric conditions, but the conditions were slightly different, you could easily introduce some error into your shot that could make it look like the BC is higher than it actually was.
Finally, you indicated that you adjusted your scope for elevation adjustments (as most LR hunters do). We all know that scopes, no matter now much you pay for them, can't guarantee perfect 1/4 MOA clicks for the full range of adjustment. This is yet another source of error that could introduce error into your assessment of a BC.
I really don't mean to pick on you. I believe you're a successful LR hunter because you know your stuff. I'm just saying that there are many reasons why our observations wouldn't necessarily agree. The shooting I did was done specifically to determine BC. None of the variables listed above introduced error in my test. Your purpose in shooting was LR hunting, in which there are far more variables involved that muddy the waters, and you might not notice it if your tables were off by such a small amount. In terms of drop, there's really not that much difference between a .580 BC projectile and a .625 BC projectile at high speed.
By the way, here's another interesting fact...
Myself and others are always on the 'G7 soapbox'. We say that the G7 BC is better for long range projectiles, blah blah blah.
Here's another example of why.
The tables that work so well for Doug use a G1BC of .625, and result in 21.2 MOA of drop at 1000 yards when fired at 3200 fps in standard atmospheric conditions. Using my measured G1 BC of .580, the program predicts 22.1 MOA of drop in the same conditions, which is a 0.9 MOA difference. Now, if you use the G7 BC that I measured, which is .297, the program predicts 21.7 MOA of drop, which is only 0.5 MOA different than the table that Doug generates using the G1BC of .625!
What this suggests is that the G7 BC is better able negotiate the large velocity drops that are common in long range shooting. When using a G1 BC, you have to have several BC's for different velocity bands, or have an average that's suited to a particular range shot. With the G7 BC, you don't have to worry about the velocity, the G7 BC stays constant and results in more accurate predictions be it drop, wind drift, tof, etc.
In conclusion, I don't think we (Doug and I) necessarily disagree about the BC of this bullet. In light of the above variables, I don't think you can fairly say that I'm underselling the BC of the bullet because the difference in drop (which is your measure of BC) is very small, and is affected by many other variables.
Again, my intent is not to say "I'm right and you're wrong". I think we're likely in close agreement.
-Bryan
