The gnasher should be your primary.
No it shouldn't and this is exactly what's wrong with the PVP side of the game. There are other weapons to use besides rolling around with your shotgun, but all anybody ever does is charge at you with them and honestly it gets boring after 5 years and 3 games.
The sawn-off and retro are not over powered, they are just an effective alternative to your precious gnasher and that's why you don't like it. The sawn-off is cheap definately, but it's far from over powered when you have to be in spitting distance to get the kill with it and as it's already been mentioned it has a huge reload time between shots, so it's far from being over powered.
If they were truely over powered they would dominate everyone and everything in the game which they simply do not.