It’s been awhile since I read the codex entries but what was the explanation of moving from heat sinks to thermal clips?
It's the "Small Arms" sub-entry in the codex that changes entirely from ME1 to ME2 (and it stays the same in ME3)
Small Arms in ME1 Codex - it starts here
and then the change is explained here: Small Arms in ME2 Codex
I'll copy/paste:
Modern infantry weapons are micro-scaled mass accelerators, using mass-reducing fields and magnetic force to propel miniature slugs to lethal speeds. Nearly every gun on the battlefield is laden with features, from targeting auto-assists to projectile shavers that can generate thousands of rounds of ammunition from a small, internal block of metal. It was long thought that personal weapons had plateaued in performance, but the geth proved all theories wrong. Mathematically reviewing their combat logs, the geth found that in an age of kinetic barriers, most firefights were won by the side who could put the most rounds down-range the fastest. But combatants were forced to deliberately shoot slower to manage waste heat, or pause as their weapons vented. To eliminate this inefficiency, the geth adopted detachable heat sinks known as thermal clips. While organic arms manufacturers were initially doubtful this would produce a net gain, a well-trained soldier can eject and swap thermal clips in under a second. Faced with superior enemy firepower, organic armies soon followed the geth's lead, and today's battlefields are littered with these thermal clips.
So in two years it's a change that makes them all switch thermal clips instead of waiting for weapons to cool down. It's a "step up your game" kind of decision that took hold after the geth attack on the Citadel, but as people have pointed out here, here and here, the lore is not always consistent and the choice isn't always popular or make sense to everyone.
