I'd love to if I had internet here ._. unfortunately posting my drawings will have to wait a few days. Using phone as modem = no good.
The butt looks weird (not the extendable bit, the part the extendable bit slides into), looks like clipped geometry instead of parts attached. Doesn't look right.
You will never ever see the rear part ingame. I decided against adding stuff to the top, thiers nothing usefull to add so it would only look akward.
Shotgun doesn't need ironsights. The rifles have them because they are rifles, they are ranged weapons. Notice how the HMG, a non ranged weapon of the rifleman, has no IS?