Dogman,
This is one of those "ask 10 people get twelve answers" questions.
I have some guns that do better one way than the other. In other words, the same load techniques don't apply across the board.
In my roller (perdersoli) I drop tube 63 grains of swiss 1.5 then compress very slightly ( about a 1/16th inch iirc). If I compress more groups widen in a hurry.
I use standard neck sizing, which NORMLLY is intended for jacketed bullets of .457 neck dia. This sets me up for neck tension out of the gate as my bullets are sized to .459. But that is not all of the story. Brass, as any metal, has "spring" to it. So when neck sized down it may be smaller than .457, say, .456. Now I have a problem. That with deform the soft slugs we use and ruin any hope for accuracy.
I have bought SEVERAL expander plugs over the years. From track of the wolf and Buffalo Arms (both great places to buy from and support!) So after neck sizing I run the proper expander to give me just right about a thousandths of an inch undersize neck. This gives good tension in this gun. (my trapdoors need crimp, more because of need to build pressure to upset the base of bullet to fill notoriously oversized grooves. that is a different story tho).
Then drop tube, place .030 fiber wad then use compression plug to depth. As I seat the bullet I set the dies to take all but the smallest bit of bell out. I read somewhere that the slight bell helps some how. Perhaps it was the centering of the slug as mentioned above. I don't remember where I read it, it just seemed to work. I DID have to take my first batch back to the bench as, like an idiot, I didn't check to be sure they would chamber with the bell still present. Doh! So I had to find that setting on the seating die to get the bell almost smoothed.
I am no champion shooter. Have had some luck when I put time into it and my mind. I tinker too much. But that gun will shoot doing all of that. All guns are different. I will shut up now.