The system to stop skill advancement when your character is more accurate than the gun seems as though it might be slightly bugged. My L2031 enforcer (dispersion 150 with mods, ammo dispersion 180) stopped giving me skill advancements once I got to 3 marksmanship, 2 handguns, which should be associated with a skill dispersion of 465. So I switched over to the L1820 Long Ranger (dispersion 0 with mods, ammo dispersion 180 again) - this got me up to rifles 5, marksmanship 4 (associated skill dispersion, 315?), at which point I stopped gaining skill again.
Honestly, the entire system there seems kind of annoying, particularly given that gun mod installation is tied to marksmanship skill. It guarantees that better mods will never be usable unless you use, essentially, sniper rifles. I don’t see what benefit is offered.