Oh, don't leave me out of the loop....what topic was that, Charles?
See posts 36, 55,
57, 59, and 66.
Keith's Post #57:
These order of magnitude or "ball park" calculations can be very useful, particularly for identifying factors that may make a difference. It's important to have a threshold to compare to. For 1000 yd shooting, the bullet drops about 500", right? A factor would be important if it changed drop by say 0.5" or 0.1%. An order of magnitude less, 0.01%, we would call insignificant. If a kernel of powder weighs 0.02 grains and your load is 40 grains, that's 0.05%, so that's significant.
For bearing friction, the 25% of the pressure that ends up propelling the bullet would be 15 kpsi, so the 0.002" variance in bearing length would change that pressure by 7.2/15k = 0.05%. That's significant, too.
Changing a 0.5 BC by more than 0.00005 would be significant. At 1000 yd, it's difficult to find things that are NOT potentially significant.
Essentially, what is being called "significant" is really "significant for further testing." Couple of points, via stories. When Steve Shelp was working up his .338 Yogi a number of years ago, he became enamored with VV N-170, as it gave such low ES/SD numbers. Sadly, it didn't group very well. When he switched to H-1000, the groups got much smaller, even though ES was in the 20s.
The whole point with velocity variations is we deal with them by tuning the rifle. Whether or not one kernel of powder, or a .002 inch difference in bearing surface is significant depends in part on whether or not you can render it insignificant by tuning.
So, while a small difference in bearing surface, and the attendant pressure difference, may theoretically be significant, it is one of the smaller contributors to velocity variations and in any case, is tuned out with general (successful) tuning.
As to BC variations: along with Dave Tooley, I got the raw data from the testing Larry Bartholeme and Dr. Oehler did at 1,000 yards with a model 43. Data available included actual B.C. numbers with each shot, which let you see BC variations. All of a sudden, we had ES/SD for ballistic coefficients.
With the acoustic target, we also got a shot plot, which let you correlate B.C. with bullet strike. A number of their tests were three to five shots, but a number were also 10-shot groups. The best bullets for BC consistency were several of the Hornady A-Maxes. With that observation, I speculated that meplat consistency was,
in mass produced bullets, the least well-controlled variable -- the plastic tips were, of course, very consistent.
That led Dave to make a meplat uniforming tool. (Aside from Charels Bailey's die & file, I believe Dave was the first, at least in post-1995 times, to make a meplat uniforming tool.) Dave also purchased a model 43, and tested the effects of both meplat trimming, and inserting plastic tips in non-tipped bullets. And sure enough, either technique brought the B.C variations way down. Remember, this was empirical testing, not calculations.
But it got me interested in the effect of mepalts, and using the formula in Vaughn's book, determined that a B.C. variation of .020 was worth 4-5 inches at 1,000 yards. That's a lot larger variation than what Keith is saying. The worst offender in the Bartholome tests was the 210 Berger VLD, but I'd hasten to point out this was at the time after Walt Berger had sold the company, and before he bought it back. As Eric Stecker stated, he & Walt worked very hard to restore the company's products when they bought it back.
Anyway, the 210 VLDs of that time were often fish-mouthed. Charles Bailey, a very inventive guy, made three sorts, using different measurements, of these bullets -- "A", "B", and "others." He could accurately predict the *general* performance of his sorts. And it turned out if he filed the fish-mouth of his "B"bullets, they performed as well as his "A" sort. The year he won Shooter of the Year he was shooting his "B" bullets, after removing the fish mouths.
OK, the point of all of this: it took a variation of .010 or .020 (on .610 nominal) in BC to have any significant effect on the target. Even the .010 variation (barely noticeable on target) amounts to a 1.3 percent change. That's a long way -- close to three orders of magnitude -- from Keith's suggestion
Changing a 0.5 BC by more than 0.00005 would be significant.
It does fit with the calculations I made using Vaughn's formula, however.
I admire & respect Keith (& Phil, too). What I really wanted to suggest, to new shooters at least, is that what's termed "significant" may not turn out to be so. As Keith said,
These order of magnitude or "ball park" calculations can be very useful, particularly for identifying factors that may make a difference. It's important to have a threshold to compare to.
From testing on these matters though, I do believe he's about one to two order of magnitude off. So for the new-to-long range shooters, I'd say while making a bearing surface sort at the .002 level won't hurt, I don't believe it will help either, at least in terms of diminishing the effect of velocity variation. I have no data on whether or not it can play a significant factor in drag. My guess is that at the .002 level it will not. It would be easy enough to compute using the JBM program, but it would take a re-testing with the Oehler, after careful measuring, to confirm.
. I'l try it with an infrared thermometer.
