I've modeled this MSBS-B rifle in my free time, took a few hours but I managed to do it. I have yet to add textures or materials but the model is completely finished. As seen in the render image above, I've also added a blur effect that can be adjusted and removed. The reference image is from Wikipedia here: https://commons.wikimedia.org/wiki/File:Msbskandmsbsb.jpg Do give my blend a chance and do leave some feedback and I hope you enjoy the model. Thanks. -Iam2busy
This model is actually quite good. Maybe a little bit of bevelling to make the edges a little bit softer. The bad thing I noticed where the flipped normals and the topology.
Many of the normals are inverted(for Example the long almost black side faces on the top). You can take a look at the normals by going in the right Window of the vieport("N") in edit mode under "Mesh Display > Normals > the face symbol. All the blue lines have to go away from the geometry. You can flip individual selected faces by pressing spacebar and typing "flip normals"(there is maybe a shortcut). Flipped Normals can have bad effects on the shading and rendering process.
(This is the point where I am not sure) By my understanding you should try to achieve faces which have 4 vertices to have a much clearer topology.
I hope I could help. :D