First of all, thank you very much for the kind words.
As for the ammo approach, I guess it would be doable, but it would require a lot of redesigning from ground up. Because of how the game works, the same ammo type for different weapons may have different properties, for example the Mauser gets bonus damage from shooter's accuracy but other 9mm pistols don't, and bonus damage is an ammo property, not a weapon property. And also there's the elephant in the room Akimbo mentioned - different guns use magazines of different sizes.