It looks like you don't argue about the change being good or bad but about the fact it's different from ME1. If heatsinks would be present in ME1 and gone in ME2 you'd complain just the same.
For me ME1 shooting doesn't have sense. Nothing is infinite but ME1 weapons act like perpetum mobile. Get you frictionless materials and shoot until the end of the world.
WillieStyle wrote...
There is no way to justify the change through lore.
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Lore? Even if ammo is like a grain of sand, the sand should also run out. It does in the hourglass.

And heat? At those incredible mass accelerator slug velocities your rifle should melt down after few seconds of firing. Heatsinks sound perfectly reasonable. It should be present in ME1 as well.