Are you talking about rings between Special-Gewehr and Lauf-Stahl on gun barrels. I think the difference between 3 rings and 4 rings is analogous to difference between Excedrin and Excedrin Migraine in analgesic isle of your local US drug store.
It's just like on those Parkers of different grades from Meriden CT all kinds of fancy smancy names on rib, but all same barrel steel.