Jellico, I would say the Marians favor Autocannons as we see many of the units they build-
Gladius, as mentioned it has a AC/10 even if it has to be imported
Gladius II, weaker IMO but it has a LAC/5, MGs and RL15s
Testudo, UAC/10, A4 and LMGs
Centurion 9-H, LB-10X, ML, and 4 RL10s
The BA was covered . . .
Now, do they import those autocannons? They used to when the Gladius was built, but we have no idea if they do any longer- heck we do not even know if they still build that Cent as a mix Prim/Modern or a all modern version. Almost as interesting for the advanced ACs is do they build their own ammo for all the options.
We do know they had purchasing agents wandering the Inner Sphere to buy discarded, old or damaged weapons and equipment that they would refurbish.