The core book talks about certain weapon upgrades being mutually exclusive to each other, and gives the examples of extended clip and belt feed. What about belt feed and fire selector?

Depends on whether the weapon is large enough that you can cram in three ammo feeders somewhere.

Logically? No.

Belt feeds are fit into the weapon. A fire selector for belt feeds would need very large moving mechanical parts to swap around stuff which makes it practically cumbersome/impossible to do reliably. Not to mention it would be a pain to handle and carry around.

Id allow it on a mounted weapon, but not a portable one.

BrotharTearer said:

Yeah, but fire selectors irl are for switching between different firing modes (like semi-auto to fully automatic), whereas this upgrade lets you load multiple magazines. Presumably you could have multiple belts going where the magazines go…?

The Bushmaster chain gun uses dual belt feeds. It should be possible to make a smaller version for a hand held weapon.