Contrary to gun shop gossip,the big reason the new stuff isn't as smooth as the old stuff is lack of work polishing, not lack of standards in Ilion.
I've Rupe's Express downstairs, as a thank you for the goose hunt I deep cleaned the thing. It has less than 500 rounds through it.
My Number 6 870 was made in 1955. It's a Wingmaster of course. And, since it arrived here in such pristine condition that the action bars had hardly any wear marks on them, it probably has less than 500 rounds through it.
Neither is as smooth as the higher mileage 870s. The TB, my oldest "Serious" 870s, and Frankenstein are like glass. My "Deer" 870 is getting there.
In fact, in a blindfold test I might have trouble telling whether I was shucking #6 or my friend's economy grade 870.
I did the things mentioned in the Breaking In New Pumpguns thread with both,so the playing field's level.
The big factors in smoothness are use and lubing correctly.