That is true, that the longer barrel of a carbine will yield increased velocity/range/performance from pistol ammo. I suppose the reason I posted the link was because even though the military is limited to what the Geneva Convention says it can use, civilians are not limited by such things. Which means that, even though your particular rifle was designed to use FMJ ball ammo, you don’t HAVE to use it. You can invest in more effective ballistic tip ammo, or hollowpoints, or partition bullets, or monolithic solids, or armor piercing, etc, etc.. just as you can tailor the weapon to the mission, you can tailor the ammo as well.
Example: I have a designated precision rifle. I hand-tailor the most accurate ammo I can build for it. However, the most accurate bullet (either Barnes or Sierra HPBT Matchking) might not perform the way I want it to in a different situation. If I want maximum expansion on a soft target, I have ballistic tips and partitions. If I want maximum penetration on a medium hard target, I have AP ammo. Now, those might not be the most accurate rounds in the world, so I have to take that into account…
The wicked flee when none pursueth..." - Proverbs 28:1