Weapons in the game come in a range of classes and in each class there are the different names weapons. Each type has its own characteristics but the performance of the weapons though is not based on what it is but on the rank level it is assigned when you pick it up. And on an RNG style setting of its damage level and possible additional Perks.
The quality of a weapon is also show by a colour coding system. which will give an idea of its overall stats relative to it level.
Grey - Worn (lowest)
Green - Standard
Blue - Specialized
Purple - Superior
Gold - High End (Highest)
The higher the level the longer it is likely to be useful to you. So a purple level 10 gun will be much better than a grey / green gun at the same level and should still be out performing standard guns for several ranks. But a grey level 15 gun may be better than it.