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I don't own an 870 so I doubt I can be much more help than other posters, what has me scratching my head is the fact that 2-3/4" and 3" shells load into the magazine with no problem, but the 3-1/2" don't. The only thing that comes to mind, is perhaps the extra length, pushes the plunger too far upwards, causing it to drag, or hang on the inside of the mag tube, or maybe the shell itself is catching on something like a burr. Again, it's all conjecture. Are you able to rack the slide to unload, without firing? If not then it may be a slightly bent forearm rail, Sorry I can't be more help.