"What's the thinking on that? Just out of curiosity? "
Well, I hate to write long things (ain't too good at it), but I'll try.
Most of us understand that bullets of the same ballistic coefficent and same velocity will have, supposedly anyway, the same trajectory wheither it's a boat tail, hp, bt, etc, or not. The critical thing to rember for this to make sense is "the same BC
", not the same caliber or weight!
The "vacuum" which forms behind the base of a speeding bullet serves to retard it's flight just as surely as does the forward resistance of the frontal area. The cross sectional area of any bullet is normally what determines the actual power of the vacuum's drag effect. A boat tail has a smaller base area but that advantage doesn't come into play until the bullet drops below the speed of sound.
So long as the bullet is supersonic, the air pressure breaks from the jacket at either the heel, a flat-base bullet, OR at the step down, a boat-tail. That means both types have the same "vacuum" pull down to that speed. But below sonic speed, (about 1,150 fps if memory serves) the air follows the boattail's step until it reaches the heel of the boattail before it can break away at the smaller base. Since that reduced base's cross sectional area is much less than full diameter the drag effect of the vacuum on a boat tail is suddenly lessened, but it only happens below that critical speed.
Or so the ballistics experts say, I can't run fast enough to watch.