You should try working on the shape....and the shading......and the coloring.... 
Well I understand the M82 could use some work, though it could be much worse...
Bear in mind that I have just begun working on this mod, and the free time I use to make the weapons is almost nonexistent.
Also, as you probably know, my shading is not that great... or even good.
But I don't see what's so wrong with the M870. At best, the shading is horrible, and the color is too dark. However, the first can be fixed with some assistance (which I am prepared to get), and the latter can be fixed with a simple brightness check. But while you say the shape is off, the ingame screenshot (which is the only source I care about) says otherwise, more or less:
