Nmn (Paul) made an over-under shotgun for an old project of mine called The Vision. You can find this shotgun in either one of the Mixed Tape weapon mods in the Weasel's Mods section (though the colors are a bit off due to it being originally designed for the Powerslave palette).
The second gun will not work reliably in the Doom palette due to your usage of cyan colors on the metal part. And on both of the guns, the black lines you're using are far too thick, making them look cartoony and noob-ish.
Also, the angle is too extreme. Next time, try making your weapon originate a little closer to the center. I'm not saying you shouldn't make angled weapons - but weapons like these, angled so far to the right, look very awkward on the screen.