Just a quick note on the 870 suggestion, I “built” my slug gun just before savage bolt guns became popular.
Bought an 870 express 12 gauge and put some magpul furniture that allowed me to raise the comb height. I put a cantelver barrel on it and a slug gun 3x9. The trigger was so bad I couldn’t get a decent group so I swapped that out for something with a light pull. I am talking variable weight between 10 and 14 pounds and gritty as a sand dune. While I was at it I did the follower swap too.
It shoots those Remington accubonds pretty well. I would say it is acceptably accurate to 150-200 yards using the BDC.
To get decent accuracy you have to use a cantilever barrel or pin the barrel to the receiver which makes cleaning a pain in the ass. So, I thought I would end up with a multipurpose gun with the cantilever but having to swap out the comb makes it fairly impractical to use it with the regular barrel without a bit more wrench time than you would like. 3” 12s also kick pretty good and the scope can bite you if you aren’t careful.
By the time I was done tinkering, I probably had over $800 in the gun not including glass. I basically have a mediocre slug gun that isn’t really good for much else. If I were doing it again, I would go with the savage bolt gun in 20 gauge. Done and dusted for less than $600 with likely better accuracy and function and much less pain in the ass.
I've got a bone stock 870 12ga with a smooth bore and rifle sights that'll shoot the cheap Winchester slugs tight enough to cover a 5 shot group at 100 yards with your fist. Not bad for me with open sights. I've got $200 in it and it came with 2 barrels.
I believe I'll stick with the 870. Lol