The classic art for a few mechs show them carrying actual guns. The Phoenix Hawk, the Wolverine, the Battlemaster.

Now, we've got rules for mechs that are designed to swing weapons. A hatchetman (or whatever) spends 1 ton for every 15 tons of the mech, and now it has a hatchet that lets its arm make physical attacks as strong as its legs. It's Kickpuncher from Community.

Could you just do something similar for carried weapons? Devote some amount of tonnage to an arm - say a maximum of 1/5 of the mech's weight - and now that arm is strong enough to wield an array of weapons that weigh up to that amount, without counting against the mech's normal carrying ability or movement and such?

So your Phoenix Hawk could (instead of having a 5-ton Large Laser built into its arm that just looks like a gun), could have 5 tons devoted to 'carried weapon capacity.' Then you've sorta got an omnipod option, where each mission you can swap around the five tons.

Maybe yeah, it's a Large Laser. Maybe it's a Light AC2 with a ton of ammo. Or a light PPC with an extra heat sink. Or a crazy hyper-beam with 5 medium lasers.

The integrated mount would link the weapon into the mech's cooling system, so it wouldn't need its own heat sinks.

Or if you wanted to build a whole new set of things, you could make rules specifically for carried weapons where they have their own cooling rate, which would maybe mean they can't fire every round?

I know it's fiddly and kinda unrealistic, and maybe the whole concept isn't really simultaneously compatible with logic and Battletech lore.