Briley offers the straight grooved rifled shotgun barrels. The shallow grooves stabilize the wads, and keep them from rotating. This makes for tighter patterns.
Years ago, a Gun Digest article described this process being experimented by the Perazzi Company in Italy. They were able to achieve 98% patterns with certain shot loads.( 30 inch circle at 40 yards.) IIRC, the entire bore was choked, beginning in front of the throat, and tightening as it approached the muzzle. I don't recall the choke being tighter than standard "full choke".( 40 "points"( ie. .040") of constriction at the muzzle.
Most shooters have no idea that those plastic wad cups used in modern shotgun shells will rotate slowly in the barrel, due to the difference in pressure against the top and bottom of the wad from the expanding gases. The wads leave the casings a bit atilt, because of the need to make the chambers and throats large enough to let the casing- of varying thickness in walls-- open up and allow the shot, wad, and gases to escape down the barrel. The larger diameter chamber allows the casings to TIP a bit downward in the chamber before they are fired. There is a short throat and "forcing cone" made into the back of the barrel, in front of the chamber, which "funnels" the shot and wad into the barrel, This constricts the wad and load of shot inside the wad/cup to the bore diameter, as they pass through the throat and forcing cone. :hmm:
The larger diameter chamber is needed to accommodate different case wall thicknesses from different manufacturers, allow the case walls to expand, and then constrict as the gun cools after firing so that the casing can be extracted and ejected from the chamber. In Self-loading shotguns, this all has to occur in a very short amount of time. :surrender: :hmm:
