Each class type is trained in using a couple of different types of weapon, and appropriately only the Soldier can effectively use all the weapon types in Mass Effect.
The base setup of each weapon doesn't change from rank-to-rank, only their power. Some are cheaper, while some can fire more shots before overheating, and so on.
That means that a Devlon Industries Raptor I assault rifle is the entry-level of that model - and a Raptor VII will naturally be significantly better.
How to get the Best Weapons in Mass Effect 1
Given that the series is largely about blasting your way through space bad guys, finding the best weapons to fight Shepard's war in the Mass Effect trilogy is pretty vitally important.