Have been doing a bit of digging around the webs, and had some ideas I'd like to hear from people more experienced than I am. @Lost River, for one, knows who he is.
It's well known and generally accepted when shooting rifles at long range, and rimfires at 100 yards, that a bullet transitioning from above the speed of sound to below it tends to generate dispersion in groups. If you can keep your rifle bullet (centerfire or rimfire) above or below Mach 1 all the way to the target, you'll generally see better accuracy than if you don't.
We don't pay much attention to this with pistols, but I wonder how much of a factor it is. For example, 9mm 115gr factory ammo is typically advertised at ~1120 fps. If it actually launches at that speed from your gun, you're basically dumping it right into the transonic range at the muzzle. A 124 grain load, depending on your gun and loading, might get there, or it might be a little under. A 147 grain load will be reliably under. At 5-7 yards, it doesn't make any difference, but some do report significantly better accuracy at 25 yards and beyond with pistol bullets if they launch well below the speed of sound. I even saw a case where a guy reported his 9mm being able to match a .45 for accuracy out to ~15 yards, but the 9mm fell apart at 25 yards and beyond.
It's also empirically reported that many/most powders seem to burn most consistently when the pressure gets up toward the upper end of the allowed range, so simply loading down with a given powder to get the velocity with a lighter-for-caliber bullet down under the transonic range may not produce good results. We tend to like faster powders for such purposes.
It seems that there may be an optimization point around the heaviest bullet weight where higher-end loads of powders that give the best velocity will produce MV around 1050-1070 fps. That should give the flattest trajectory that won't have to pass through the transonic range, theoretically enabling the greatest accuracy at longer distances, while also delivering the greatest power factor consistent with that accuracy. Various of people's reported pet loads for a variety of cartridges seem to be right in that speed range, not 75 fps higher.
Just picking 9mm, load data suggests a 135 grain bullet should probably be maxing out around 1050-1070 fps. That doesn't land near any specific power factor threshold for competition, but it may be an optimization point for hunting, for example.
Not really sure where to go with this, other than it being one more thing to think about when working up general-purpose loads.
Any thoughts?