Wallabing, You say the bluing wore excessively in the locking area of your 870 Express. The Express isn't blued, but has someting similar to a Parjerized finish. I assume that is what you meant.
As for the wear, is it possibble that you didn't have the magazine cap tightened down (whatever you call the magazine cap) or it loosened up.
The 18.5" barrel has a spring loaded locking pin in the front of the barrel lug that allows the mag cap designed for it to lock down as cut outs along the edge of the cap ratchet down on the spring loaded pin. I assume that you are using the mag cap that came with an express equipped with a long barrel.
The long barrel cap may not catch on the detent pin of the short barrel. Still, if you haven't modified your Express detent system, which is done if you go to an extended magazine, I believe it would still catch on the cuts on the inside face of the cap that came with your Express.
Then again, it may merely be that you didn't tighten you mag cap enough when you put the barrel on.
I hope this is more helpful than confusing.
