Boat tails will reduce subsonic drag and are at their most efficient at speeds of about 1050ft/sec. They also reduce stability, so spin rates need to be slightly higher than necessary for a cylindrical based slug. They will be most effective in long range shooting, but they will also reduce cross wind effects at any range.
The above assumes the boat tail is visible to the air and that it has been designed properly. The vast majority of boat tail slug designs I have seen have not been properly designed and are almost certainly doing nothing, if you are lucky, except add a little extra mass for a higher sectional density. If you are unlucky, the badly designed boat tails are drastically reducing dynamic stability, which will give flight problems.
On most of the designs the angles are far too high and the boat tail lengths are often too long. The angles the air on slugs can turn through are very small, and the air simply cannot turn enough to follow the majority of tail designs. The result is that the air breaks away at the end of the cylindrical section, just as it would if the boat tail was not there.
There are some new Bobs Boat Tail designs available for self casters with boat tails designed to work properly based on what I have said above.
