[Daekar]

As somebody who has spent quite a lot of time researching how far the 9mm can be pushed, I totally understand the reasoning behind your question. The fact is that the 9mm CAN be pushed much further than most people realize by handloading and using a rifle length barrel. As is usual with cases of limited capacity, it is best to use lighter bullets when trying to make the most of slower powders, and in single-shot rifle, light bullets can break 2000fps without pressure signs.

However, in a semi-auto the question is a bit different, especially when using factory ammo. In general, though, the recommendation to stay away from the heavier bullets stands IF you're trying to milk the barrel for all the muzzle energy you can. There are exceptions, but generally speaking the 147 grain bullets will not benefit much from the extra length. This makes sense when you consider that most of them are designed to remain subsonic for use with suppressors. +P varieties may violate this general rule. Since neither the 124 or 115 grain loads are subsonic, they are generally loaded with powders that allow more acceleration in a longer barrel. If it were me, I would see which of the two weights my gun shoots most accurately at the ranges I'm concerned with, and shoot those. The effective range of the 9mm carbine is longer than most give it credit for, but no really long enough for the slightly higher ballistic coefficient of the 124 grain bullet to matter. They both have enough free case capacity to allow powder loads which can take advantage of the extra barrel while still cycling the bolt correctly.