Actually, a boattail doesn't make that much difference until you reach subsonic which is WAY out there, depending on B.C. of course
I'm new at this so I'm trying to learn. Reading everything I can find. Trying to follow along here and understand the discussion. Litz's book is the best reference I've found so far. Looking in the book, I'm not so sure that's true. I'm sitting here looking at the graph, figure 2.7 on page 23, in Litz's book. It compares the drag for the G1, G7, and a modern long range bullet. G1 has higher drag everyplace.
Near as I can tell, a boat tail won't matter much at less than 300 yards for sure. But as the range gets longer it makes more and more difference. Especially with bullets on the lighter end of the spectrum where a small gain in BC is a big percentage.
Surprisingly, there is also data in the book that shows some very low drag bullets don't do well subsonic - they get rather unstable. I think one could say flat base bullets actually do better subsonic than some low drag bullets because they are more stable (center of pressure is less aft of center of mass compared to long nosed VLD type bullets).
That said, 1800 fps, which is the stated minimum expansion velocity for quite a few modern bullets, is supersonic so subsonic performance is academic since they shouldn't be used at ranges where they will be subsonic for terminal performance reasons. That makes the trade on the side of higher BC vs subsonic instability a good one since subsonic really doesn't matter if the bullet won't do the job after impact.
I think three things (not in any particular order - I see them as a chain, not a pyramic) matter quite a bit when it comes to hitting game at long range:
One is at what range does the bullet drop below it's effective expansion velocity? More BC extends that range.
The most important factor relative to making a good hit, when hunting is wind drift. Range can be measured, wind is considerably more random and measuring it accurately over the distance of a long range to target across verying terrain is a practical impossibility. More BC reduces wind drift which increases the hit probability - which tells me that all other things being equal, picking the bullet with higher BC is better for hit probability.
Then there is what the bullet does after it gets there. I don't really understand the trade off between terminal performance and BC very well. It seems to me this is an important thing to understand. I'm liking how low drag bullets shoot, I need to understand more about their terminal performance. At the moment my practice is to respect the stated minimum velocity but I'm not sure that is enough.
Sitting here thinking about it in real time my tentative conclusion is that the two most important things in bullet choice, in order of importance, are suitability of terminal performance for the game to be hunted at the intended max range and ability to shoot them accurately out of the rifle in question. Ballistic coefficient is more or less important depending on the range, and that depends on the game.
Which brings me back to the original question, where does BC get important given game and range. My answer to that is when the wind drift "uncertainty" is over half the the size of the kill zone on the animal. Not the expected wind drift which may be several kill zones in magnitude but which is compensated for by estimating wind and making sight corrections, but the uncertainty in wind drift. i.e. one measures the wind and concludes the wind drift will be 40 inches but the uncertainty is a foot or more given terrain variations. Measuring the wind will reduce that uncertainty. Higher BC will reduce that uncertainty. But I think the range at which it becomes important varies with the size of the kill zone on game in question.