My original BBT's were designed with a boattail similar to what Corbin uses.... It has a 0.015" rebate because anything thinner and the die will break during the swaging process.... I didn't know that at the time.... When British Ballistician Miles Morris examined those designs, he said that between the rebate and the taper (again, similar to Corbin) he doubted if the flow was reattaching before reaching the base of the bullet.... He suggested reducing the rebate and taper, so that the flow could reattach before reaching the base.... I went through all my designs and changed all my Bob's Boattails as per his suggestions.... Only the rebate changed in the longest designs, but in the shorter designs the boattail ended up a much shallower angle as well.... The first slug made to the new designs ended up being the 106 gr. 7mm made for Scott Hull, and the new design made a big improvement to the Ballistics Coefficient in the 900-1050 fps range, and is at least as good at lower velocities.... There are a bunch of new BBT's in the pipeline from NOE made to the new designs.... The reason I am giving the detail about the similarity of my original BBT's to the Corbin design is that I have shot a few .172 cal Corbin boattails, and their BC is no better, if as good, as a similar weight NOE slug with a gas-check base.... On the other hand, they don't seem to need a faster twist to be stable, as a boattail in theory should require.... My conclusion is that particularly in the smaller calibers (where the 0.015" rebate is a larger percentage of the diameter) I don't think the Corbin BT is working properly.... Again, just a hunch, based on limited testing.... but on the other hand, NSA, who used to make quite a few boattail slugs, have dropped most of them, and in particular the smaller and shorter ones.... I never asked Nick why, but I'm guessing he found out the small Corbin BT's weren't achieving the theoretical benefit.... Bob
Well, not to discourage casting of BBT's, but if swaging machines can make wasted pellets, they can also make boattails (or conventional slugs) with a reduced diameter center that doesn't touch the lands, and should be able to make a much smaller rebate than the axial Corbin dies by using a split die like they do for pellets.... All it takes is the desire and tooling, IMO.... Of course you lose the adjustability of the axial dies, where you can change the length and weight by adjusting the position of the base ram.... You would need a separate set of dies for each slug weight.... just like pellets today.... Bob