>>101271196
The weapons in the original game used a mass effect field to accelerate the projectile and make it hit hard, kind of like a railgun, but they shaved a small amount of metal off a metal block to generate this projectile. That's why ammo was effectively infinite and only heat was a concern.

EA bought the developer, so they had them completely redo the gameplay as a generic trendy cover shooter of its time, which means they put ammo back in for no particular reason. This is justified by saying overheating was "solved" in-universe with disposable heat sinks, and somehow all old guns were disposed of in 2 years and all modern guns refuse to fire without a clip even though technically they have thousands of shots in them.