In one of his interviews, Mike Walker remarked that the key to the quality of the Remington .22 benchrest bullets was in large part due to the sheets the jackets were drawn from. He made the point that DuPont was big enough, had enough clout, that they could reject any material not to their specification.
Those Remington .22s were probably some of the best bullets ever made, custom or factory (Remington lost money on them).
Today's custom bullet makers are dependent on J-4 jackets, though from what I'm told, when Sierra gets a good lot, their jackets are superb. Problem is, their tolerances aren't as high, so those special lots don't come along as often. (They do argue that the difference can't be shot).
So the rest of bullet making is (1) design, and (2) pretty straightforward mechanics. Good dies, figuring out the lube, developing a "feel," etc. As always, good design is a compromise between performance and repeatability.
As far a the BIBs go, R.G. Robinett is a good enough bullet maker to have covered both (1) and (2). Like everybody, he's dependent on the jackets he receives. His design leans toward the repeatability side of things -- no VLD performance, but no VLD quirkiness, either.
[Aside: In the interest of full disclosure, R.G. is a friend of mine, but I'm not trying to tout his business -- he's already got as much work as he can handle, and got backed up from the stroke he suffered last fall. We had an email exchange last week, and he reports what with the PT he's done, he's in the best shape he's been in the last 20 years. Even better, the stroke was caused by something not apt to repeat.]
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I wasn't aware of the "super 6mms" at Pennsylvania, but if we pay attention to Mike Walker (usually a smart thing to do), "perfection" still depends on the sheeting from which the jacket are formed.
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Now, how about getting back to the .300 Ackley... Esp. with 190 grain bullets and either 4831 or 4350. Anyone got any more wisdom they're willing to share?