You've already thought about AK-receivers, so you'd obviously need a distinction for ARs between NATO and AK. Perhaps even more (chinese stuff?). Same for sniper rifles (NATO vs. WP bc of scopes (PSO-3) ).

How would you handle new rail system vs. 'unrailed' guns? I mean, there might be attachments that fit on a M16A4 but do not fit on a M16A1.

Magwell adapters only fit for a specific caliber. How would you handle that?

Foregrips wouldn't make sense on single-shot shotguns, but would be useful on automatic shotguns, so you'd need 2 flags there...

SMGs that already have a built-in silencer won't accept a new one, so you'd need new tags there. This is under the assumption that the silencer effect has been calculated into the weapon stats, if its just a inseparable default attachment than your system is ok.

Don't get me wrong, I like your idea. I guess what I'm saying is that once this gets adopted, people will soon find guns that are in some way distinct, and you'll therefore need a lot of new flags. If you add new flags afterwards, you'll have a ton of work.