Thoughts
I'm speaking of Smooth bores only, using Foster type slugs. Some call them rifled slugs. For Simplicity sake and some is applicable to rifled bbls/ or smoothbores with rifle choke.
It is true each bbl/ choke has its own preference. Even bbls coming off the same assembly batch often times will have different preferences in loadings, be it pellets, or in this discussion - slugs.
Also different lots of ammo will vary - hence the suggestion once you find a loading - run back to store and buy a bunch of that same lot nuimber.
In general, for me over the years, Rem bbls seem to do better with Fed loads with Win being next.
Barrel length does not make a difference - ONLY to add longer sighting planes do assist with sighting when using plain bbls, vent ribs with front bead only. One the vent ribs, adding a mid bead assists in not canting the gun...have gunsmith install and tweak to fit your loading.
Fixed Choke bbls only-
If acccuracy is giving fits, make sure the muzzle is not dinged. If so have a competent smith correct.
Screw in choke folks-
Internal chokes folks make sure muzzle is not dinged, hence one reason I prefer external knurled chokes.
External knurled folks, see if choke is dinged.
Internal/ External : Make sure the choke is concentric, will not only affect slugs , also pellet loads.
Here is a rare thing, affects pellets and slugs. Make sure where choke fits into recess of bbl - there is NO lip. By this I mean the screw in choke should be flush at least or a itty bitty below where is snugs up to inside ridge of bbl where threads are cut.
If the wad catches this , even ever so slight, this can 1) throw pellets/ slugs off. 2) with heavy loads, especially slugs, catch that lip, ruining choke, threads, end of bbl and near muzzle portion.
It is a given if Bubba has cut the threads and installed a choke - very often nothing is concentric and nothing exiting that bore will go anywhere near it was supposed to.
Sometimes only a couple of thousandths in constrictions make all the difference in how a gun groups slugs ( or pellets). Aftermarket chokes with the smallest increment in thousandths of availablitiy allow one to tweak to fit for their gun , loads and needs.
Just because a 12 bore is "supposed" to be .729 does not mean YOURS is.
5 thousandths of constriction in a bbl .729, .735 or .737 makes a difference in each of these.
Gun fit to shooter, proper form, also play a part.
Brister was right - there is art and science to this shotgunning.
HTH
Best of luck to all...
Steve