Interesting. All of my 870's are older guns. My main teaching gun, Humble Pie, is an 870 Express purchased around Y2K. It's been upgraded and most crucially the recent manufacture rifle-sighted Police barrel on it has had the forcing cone opened and the whole thing from chamber to muzzle has been honed. My other 870's are all vintage Wingmasters. I've fired a lot of #4, #6, and #7 Winchester Super X birdshot through my 870's without trouble.
Remington's failure to properly polish at least the chambers on their barrels is disconcerting.
I've seen the same from Mossberg a fair bit, too.