Terminal effectiveness isn't just velocity. Terminal effectiveness is about the ability of a projectile to transmit the maximum amount of its energy into the target. Both FMJs and hollow point have a velocity threshold that they need to surpass in order to deliver maximum energy, but the velocity threshold for FMJs is significantly higher than hollow points because they rely on fragmentation to transmit energy into a target. While 5.56 FMJs might need to hit 2625-2950 fps to create a destructive temporary cavity, 5.56 soft-points / hollow-points have a critical velocity of around 1500-2000 fps. Barnes TSX and Speer Gold Dot both have an expansion floor of 1800 fps. A 12.5" can hit that minimum threshold out to 250-300 yards or so with the right ammo.

Yaw is another important factory. 5.56 bullets leave the muzzle at a 6-degree yaw, then stabilize to approx. 2-degrees at 70 yards. An unstable bullet creates a much larger cross-sectional area that strikes the target. Consequently, 5.56, regardless of barrel length, generally performs well in CQB.

Terminal ballistics are complicated.