I have one on my M4, along with the 8" and 6" kits. It's still loud as **** with the 8" and no ear pro, so I didn't bother with the 6". It's much quieter in the stock 12" form, but you'll definitely notice the weight on the end of your gun. I may try the 10" kit for dove hunting, or just stick it out with the 12".
Also according to a silencershop review, the M4 is louder as a host, probably due to the bolt unlocking faster than the mossberg 930 they tested.