That's really poor critique.
It looks plenty like a plastic weapon.
I dont think he means that it doesnt look plastic, i think he means that it doesnt fit blockland because, well, how would a blocklander look while holding it? If the gun was for a game that uses semi-realistic human models then it would fit really well.
Lets look at the shotgun featured on the front page:
Much like a blockhead himself, it has round stubby proportions, and an overall toon-like style.
Your shotgun looks as though it is modeled and proportionally scaled to a real-world weapon.
This is why it doesn't fit, because blockheads are not proportional to real-world people. If anything they would be more proportional to a toddler.
My suggestion would be to shorten all of the main components and exaggerate the details.