Went to the range today with my 12 ga. '88 or '89 era 870 Express with an 18.5" Mossberg barrel, cylinder bore. I bought the barrel 20 or so years ago, never used it.
I fired 15 rounds S&B 00, 10 rounds Winchester low recoil LE ranger slugs, 5 rounds Federal tru-ball slugs and 5 rounds Winchester 00 Buck of unknown vintage. All went "bang" and landed where intended, more or less.
I started with "load one, shoot one." and then worked on follow-up shots. My primary focus was on safe and consistent weapons handling and getting somewhat objective observations on patterns and slug groupings.
Overall I like the 870. The solid rubber recoil pad will get replaced with a 'modern' one. My technique needs some work but that's what I expected.
You lost me right after the "put a Mossberg barrel on a Remington".