I am assuming the picture which you are talking about is the one posted by Aragorn50, in this thread. Compare the length of the engraving foot-print of the ZA to the Scenar. The difference is notable. We agree that the length contributed by the shaft does nothing to improve form factor.
In a world where spin stabilization does not complicate matters, this is what a bullet would look like;
File:Sears-Haack.png - Wikipedia, the free encyclopedia
There would be no
shaft on the bullet made in heaven.
The VonKarman equation takes this feature into account, as it defines a minimum-drag nose constrained by a diameter (such as a gun bore). I use the VonKarman variation of the Haack formula in calculating nose profile to within less than .0005" over the entire length. VonKarman did not address minimum drag on a spin-stabilized tail.
I have, and to my knowledge, I am the only one that has bothered to even look at the problem. You will not see the solution in the photograph, and you are correct that a long tail, in itself, will not aid a spin-stabilized projectile... in fact it will diminish BC due to lack of stability.