That 120mm probably would be abstracted as some form of Artillery. Tough to say which one since I am having difficulty finding actual performance data for it.
I'd be more than happy to abstract that Bofors 57mm L/70 as just a Heavy Rifle Cannon that as a matter of fluff fires bursts(much like any Battletech AC) based off the figures on the wiki.
Yeah. Wiki doesn't have a lot of info on it. :( The point though was that "ancient" autocannons are going to be bigger than machine guns.
Then Rifles wouldn't be single shot weapons and the fluff was specific about that to make them separate from autocannons.
I wouldn't worry about trying to account for every possible iteration of ancient weapons and attaching stats to them. Even I didn't do that in my "Battletech 2" project.
Sometimes it's okay to just abstract it away.
I don't plan too. Just a representation is good enough for me. Not that I'd complain if Herb published stats of weapons he did for XTRO:1945. ;)
Thing is we want to improve Rifle Cannons. If we know how Autocannons improved we can apply the same to the Rifles. So far we know they've become more reliable and gained some ammo. Their damage should also be -3. If not then the -3 for Rifles is just a needless nerf. If ancient AC ammo is less than the Primitive Prototype ACs by the same amount then the ammo per ton has doubled between ancient and standard. The ancient ACs should also be less reliable than the PP ACs.