I have found that buying slugs from the same lot # can help.
Also, esp on 870's..........the tightness of the bbl to receiver fit can be a bit sloppy. On one Police Magnum I used two layers of masking tape on the flange, rubbed down , to take up the slop (bbl then tapped into receiver with wooden block).
I don't disassemble the gun until after deer season.
Mag cap is torqued on with leather and channel lock, not too tight but more than finger tight.
Also, I clean at a certain amount of shots and foul for two and then am "good to go".
Test to see what your rig needs.