Pros:

(These are for both the M4 and the Chrome shotgun)
Very enjoyable to use! Means I actually use the M4 and the Chrome shotguns sometimes :)
Powerful and accurate, but not overpowered!

Cons:

Being the fussy guy I am, I dislike the way that you do not have an perfect number of shells for magazine size. If you were able to add +1 to the total no. of shells (eg. 91 shells total) for this and the chrome shotgun, I would be over the moon. No points deducted though :P

Conclusion:

I installed your tight shotgun mod for both my pump and spas shotguns, and your slugs for my chrome and m4 shotguns. They have made using shotguns much more fun (not to mention tactical - to take the slugs for taking out specials at distance, or the tight spread shells for close quaters?).
Overall, very well done! EXCELLENT!