ORIGINAL: SWThomas
I'm willing to bet that 9 out of 10 870's that have jammed are due to sub-standard quality shells. My buddy and I were at the trap range one day and he was shooting Winchester sport loads. The cheapest thing Wal-Mart had. He experienced about 6 jams. I gave him a box of my Remington shells and he shot the whole box without a single jam.
The Winchester Universals are known for being extremely cheap (junk) and jamming in semi autos. However, this should not happen in any pump. If the chamber area of the 870 Express were not so rough it would not do this. The simple fix is polishing the chamber (I always polish to the forcing cone) and shooting whatever you feel like.