Thanks Jimro, for that referral; it was very interesting. This was a test of several bullet seating dies where all other variables were removed (as much as possible). The Redding die got the best unfired result and its bullets shot a very good target. The RCBS, which seemed to do a respectable job in keeping the bullets concentric, shot the worst target.
At $165/die, the Redding Competition Bullet seating die is pricey so I would probably get one to try.
I shoot two rifles the most; .257 Weatherby Mag and .300 Win Mag. I can say that full-length resizing of the .257 is almost like pushing hot steel into butter; very easy. The .300 takes lots more force; both to size the case and then to pull the sizing plug through the neck. Of the two cases, the .257 has a very long slender neck whilst the .300 has a very short stubby neck. Of these two, it seems to me the .300 would be more likely to have bullet-seating issues (the neck experiences greater forces during sizing and the shorter neck is less able to hold a bullet straight). Hence, it would benefit most from the Redding die.
What think ye?