I think the gunsmith, however cool, is inherently flawed and can't really be balanced. Adding even more options per slot will exacerbate the issue just as much as going from 5 to 10 slots did.

Basically any game where you have massive attachment variety has severe balance issues. From MP games like CoD, Tarkov, Insurgency Sandstorm (esp with ISMC/project arbusto mod) to even SP games like Ghost Recon or Payday 2.

You always have dozens and dozens of available attachments, but only a select few are actually viable, and your guns usually end up looking like doodoo if you want them to perform well.

I think the only proper way to do a gunsmith-like system is to have most options simply be visual options. Like instead of having 5 different barrel/handguard options all with different stats, you simply chose between default, long or short barrel, which actually affects weapon stats, and then the handguard is purely visual and you can choose whichever looks best. For stuff like grips, muzzle devices etc, you just have simple options: Compensator or Suppressor, then choose the skin for it. Straight grip or angled grip, then choose the skin for it. No more having 6 different Foregrips but Merc foregrip is the objectively best under any circumstance.