Pros:

A very refreshing look to the plain M4
Excellent sound
Looks awesome and seems more useful

Cons:

Seems kind of out of place compared next to other weapons

Conclusion:

A excellent weapon redesign, that not also looks cool, but sounds realistic. My only complaint is really that the other weapons don't seem as good now, and it would be really nice if you could create a weapon pack to redesign the sound and look to some of the plainer weapons. Good job!