Checked the code, the targeting bionic actually does work around dispersion, it lowers effective range to the target. Effectively this is similar to lowering dispersion across the board, but it can’t go negative. This is barely justifyiable considering it’s assumed to be compensating for weapon, handling, sight, target motion, everything.
A scope simply helps you aim, it doesn’t help you lead the target (perception, skill) or hold the gun steady (dex, skill), so the effect is to reduce the dispersion of the gun itself, which means it caps at 0. It also can’t compensate for poor ammunition, so the best you can do is bring the weapon’s dispersion stat down to 0. (it can compensate for other gunmods though, so if you have a silencer and a scope they can cancel out.)