A common -but not necessary sound- practice for manufacturers of economy doubles is to retrofit rifled bbls. on an existing shotgun receiver (mostly 20 ga.). I've known some to shoot loose in but a few hundred rounds. While they may start from the same rough forgings, this is where receiver similarity ends as machining and heat treatment are vastly different in order to cope with rifle pressures. Here's a pic from the receiver of a FN-Browning B25 shotgun (L), next to the one from a FN-Browning CCS25 o/u rifle (R) :

It's obvious that the rifle receiver is reinforced by having thicker walls and no cirdular cutout at the bottom. There's no such thing as a quality AND not expensive double rifle...