A Mossberg 12 Gauge Model 835 is overbored to .775". Don't shoot a slug from one of them - a chance of the slug getting sideways in the bore and possibly sticking.
As far as the Goose Gun goes, The only choke claim for them I've ever seen is Full Choke. That's 0.40" of constriction. Any Foster rifled slug can be fired through a full choke with no problems. I've seen a measurement of 0.695" for the choke on a Goose Gun, which would indicate a slight overbore from the 0.729" nominal bore size to 0.735", not enough to pose any danger from firing slugs.