if you have the type of 870 with the plastic spring retainer that pushes in and turns 90 degrees you will need to grind out the detent in the magazine. I just used my dremel to grind it out. Then you will have to get the "old" style spring retainer which will work great. What happens is the plug has a groove that allows it to be insterted past the detent, with the extentsion the plug gets pushed past the detent and when it comes back down it can get hung up on the detent. play with the plug that pushes the shells and you will see what I am talking about.
I didn't get the 10 shot from TAC, it was from a place in Arkansas. Anyway if you have anymore questions let me know.
Dave