Unfortunately, in this case, your simple science is misguiding you. Boat tails produce destabilizing aerodynamic moments about the CG due to giving negative lift when yawed to the air flow, so you require higher twist rates for the same stability factor. Coupled with this is the fact that the boat tail also increases the boundary layer thickness, which will increase Magnus moments, leading to a reduction in dynamic stability.
As for drag, the largest single component of drag for a slug with a reasonably shaped nose is the base drag. It is a well proven fact that turbulent boundary layers at the base of a projectile reduce the base drag compared to that which is produced by a laminar boundary layer. The turbulent boundary layer does this by increasing the mixing of the air in the low pressure area behind the base, increasing base pressure and thus reducing base drag. So, while you may obtain a small reduction in aerodynamic skin friction drag by keeping a laminar boundary layer, you will create a much larger increase in base drag with a laminar boundary layer compared to the turbulent boundary layer. This is basic aerodynamics, as shown by the tests on sphere drag, amongst other things. Slugs and airgun projectile complicate things further by flying at relatively low Reynolds numbers, which can cause the complications above.
