From a gameplay perspective I dont even understand why a long barrel is even here, its not like we are shooting at ranges that make it that much of a better option, if at all better. Majority of engagements are well within the hitscan ranges of most of these guns.
Compensators/flash hiders and other attachments seem more reasonable since all they need is to screw it on. Guns like the AKM and AK-74 should have the "compensator" come standard since well, they usually come from the factory with the "muzzle brake" on it similar to how the M16/M4 family should have the Flash hider as standard.
I wouldnt want to see a visual change when using the "long barrel" since it would make some guns look very unrealistic to its actual counterpart. I would hate to see someone waving an AK-74 with it, I could imagine how off putting it would make me since I have sitting next to me. But that's just me