My bet is that they are trying to mimic what a customized real shotgun would look like.
If you elongate the barrel, then you need to replace the barrel completely, and if you elongate the barrel fully, then you will have no space for hefty chokers.
however, a magazine extension can be added for one or two extra shells.
(I'm not in any way saying that I would customize a shotgun like that without getting a longer barrel or hefty choke)