I shoot an Remington 870 20 gauge with a rifled barrel and have always used the Winchester BRI slugs with it.
They have an hour glass shape.
The 20 gauge BRI starts out as .40 caliber,
When I recovered a slug that traveled almost full length diagonally on a quartering shot. it expanded to about .64 caliber, which is about 33% expansion.
They also shoot great in an H&R heavy barrel 20 ga. slug gun which is known for accuracy.
Winchester teamed up with the independent BRI a long time ago and then may have bought them out because their slugs were that good.
I can only imagine how well the heavier 12 ga. slugs would perform.
Read all of the great reviews for Winchester BRI slugs--->>>
https://www.basspro.com/shop/en/winc...ugs-shotshells