I have an 870 Police Magnum. From the time I got it, it wouldn't feed from the magazine. I consulted a shop manual and started comparing the fore end to those on other Police Magnums. I discovered that the fore end tube assembly is different than those on the other guns. Specifically, the left action bar, which cams/depresses the left shell latch to release the shell from the mag, is the problem.
What I will call the "tab" that actually depresses the shell latch isn't far enough along (towards the rear of) the action bar to completely reach the shell latch when the slide is fully in the rear position. From the rear face of the actual tube part of the assembly to the end of the tab on the left action bar is only 31/32" or so, where on the assemblies of the other guns it is 1.5" from the rear face of the tube.
I see no evidence that anyone did a chop job on this fore end tube assembly. Are there two different fore end tube assembly sizes? I guess I'm going to be shopping for one, and I want to make sure I get the correct item.