[Noise]

Would it be reasonable to assume that you could adapt firearms of a reasonable size and weight to replace a lower arm or hand using the modular cyberlimb rules? I didn't find any discussions about this on this board and Augmentation only seem to mention tools or trick-objects like the skimmer disks. Anyone used this in their games and have any good house rules for this situation?

It's extremely outlandish, yes, but it was the first thing that went through my mind when reading the rules.

[SpatulaODoom]

I'm curious to know what sort of advantage you see this providing.

[Noise]

Absolutely none, other than not being able to drop the weapon.

[SpatulaODoom]

Then check out the Weapon Mounts in the Augmentation book, page 43. That seems to be pretty close to what you want.
Toss an external mount on one of your modular cyberlimb arm and you're pretty much got that, while still having a hand.
You could even ask your GM to house rule it into a forearm instead of a full arm, since it just fits.

[Noise]

Hmm, almost. But that's limited to SMGs and smaller though. A large-framed troll should be able to carry a LMG. A sufficiently psychotic (read: Z-zone redmond barrens gangbanger) human should realistically be able to carry a battle rifle with some strength modding on the arm + a cybertorso. If you shortened the barrel at least so it wouldn't stray into the ground. And the essence -5.9 Cyberzombie Troll airdropped to defend a secret facility in the middle of the aztlan jungle could probably carry a Panther AC without bigger problems.

[SpatulaODoom]

The articulated weapon arm, which requires a cyber-torso and is an entirely separate purpose built weapon carrying limb, can handle up to a light machine gun.
Beyond that I think you're getting a little silly with the idea that people "should" be able to integrate a two handed weapon into one cyber-arm.

[Noise]

I'm an inherently silly person, and so is the concept of cyberlimbs which in fluff are described as containing gears and actuators while clearly running off of body glucose. But you have a point there regarding firing large weapons with only "one hand". Luckily, there's a rule for that @ AR 162: -1 die for trolls, -2 for everyone else. And there we also have the rules for requiring 8/8 Bod/Str to fire a heavy weapon without a gyro. It doesn't specify that it has to be a full gyro harness, though - but allowing for a cyberarm gyrostabilizer under these circumstances seems to defy physics somehow.

Do I remember wrong or did I see a gyrostabilizer weapon mod somewhere? It doesn't seem to be in Arsenal though.

EDIT: Admittedly, the text says "carried or fired" so it rather unambiguously refers to a full gyro harness. And resisting P/2S every time you pull the trigger, even with a trauma dampener and those nail-yourself-to-the-floor cyberfoot mods seems a bit excessive for the benefit. On the other hand, a sawn-off battle rifle isn't anything to sneeze at either. Especially not with an ammo drum and the FA mod.