I have the Speedfeed IV-S on my 18.5 inch 870 Police Magnum. I orginally had this stock on my 20 inch Police Magnum but replaced it with a Speedfeed III magazine stock. On the 20 inch gun, the shorter stock really through the balance off and the gun felt "nose heavy". Incidently, the 20 inch 870 with the Speedfeed IV-S is nearly the same length as the 18.5 inch gun with a Speedfeed I stock. Now that I have the 18.5 inch Police Magnum, I don't need to make my 20 inch gun shorter. The Speedfeed IV-S on an 18.5 inch gun is excellent. It makes it much more compact and manuverable. The SWAT butt plate is brutal on the shoulder, but it reduces lenghth even further. As others have mentioned, I too had problems with the comb hitting me in the cheek.
Its extremely painful, and it feels like a punch, only harder. I had to adjust my cheekweld and move my face farther towards the butt and the problem went away. Its a great way to crack your cheek bone or develop a flinch. Every shooter has a different stance/cheekweld, so you may not notice it, but everyone who has shot my 20 inch Police Magnum with this stock has experienced the same phenomenon. It doesn't prevent you from shooting effectively, its just painful and in a deadly force scenario you wouldn't even notice it. I have never had this problem with the Speedfeed I or III magazine stocks, only the IV-S. I feel like the SpeedFeed III is a more universal and more comfortable stock and unless you need the length reduction*body armor* you will be better served with a Speedfeed III. Other than that, its an excellent stock and very durable.