The reason why the flat muzzle works for the default gun and rocket launcher is because it fits with their basic low level of detail.
Your gun however has a slightly higher amount of detail, which makes the flat muzzle look weird and out of place. Adding an actual barrel will give it an even level of detail.
All that was given was constructive criticism to help improve the model. Nobody was trying to bash you or anything.
Edit:
here is an example of different barrels, ascending in detail levels, and of course poly counts.

The far left is like on your gun. Completely flat and the black makes up the entire front face.
Next is also completely flat, but it has a rim.
The next one is the same as the previous, but the vertex in the middle of the circle has been pulled inside to give some depth.
Finally the last has a whole new cylinder inside.
Even upgrading to the second example barrel with the rim around it would make your gun look a whole lot better, without adding many polys at all.