As one who owns many shotguns (hunting, competition, testing), I have some thoughts regarding backbored barrels. No great revelations here, but something to consider when reading about the pros and cons of backbored shotgun barrels.
Probably the most often stated advantage for backboring or purchasing a gun with a backbored barrel is that backbored barrels help give better patterns and reduces felt recoil. Let's look at some particulars.
SAAMI dimensions for 12 gauge shotgun barrels specifies .729" as Cylinder bore (manufacturing tolerances +- .002"). This has been the standard bore for many years and many thousands of guns are equipped with these barrels today. But what about factory shotguns that come with "backbored barrels"? Is there a "standard" dimension out there that all manufacturers comply with?
Unfortunately, there is no standard for backbored barrels and with little agreement as to what the bore dimension should be for backbored barrels. Bores range from .733" to .745" or more (the more metal you remove, the lighter the barrel, but also the thinner the walls). As a manufacturer of shotgun chokes and a believer in patterning your shotgun/choke/load combination, I have recovered hundreds of shell wads and recorded the measurements and condition of each one. These wads were recovered after firing through factory and aftermarket non-ported choke tubes, ported chokes and, of course, BlackBore chokes.
An unfired wad removed from a new shell of common, popular shotgun ammo measures about .715" at that point where the petals merge into the body of the wad and .695-.700 for the body itself. When a shotshell wad/shot column is fired in a .729" barrel, the expansion at the wad base is still about .695"-.700", the waist is running about .710" and at the junction where the petals meet the rest of the wad has expanded out to .729". This was especially obvious when shot from the barrel with the BlackBore choke where the blades in the ports mutilated the base, sides of the wad and the petals. It was not so obvious when examining the wad fired through non-ported choke tubes as there is little damage to the wad.
What happens in a backbored barrel? Using the same ammo and recovering the wads, the base and sides of the wads showed expansion to the backbored barrel inside diameter. The petals of the wads are deformed to a greater degree because of the greater amount of expansion. This could indicate a greater potential for pellet deformation (which doesn't help the flight of the pellet to the target).
What's the bottom line? All shotguns tested with backbored barrels gave good patterns using extended conical/parallel chokes. A backbored barrel shotgun may not be the great advantage that so many think it might be! By letting more of the combustion gases escape around the wad/shot column during its journey up the barrel, there might be some lessening of felt recoil, but that is very subjective and hard to prove. I would say that a ported extended choke releasing gases near the muzzle of the barrel would help to reduce felt recoil more, but that is just an opinion. Is there a more likelihood of pellet deformation with a backbored barrel? There are indications that this might be happening, but it is probably not enough that the shooter would recognize the difference from one pattern to the next. Something to keep in mind!
Also, keep in mind that BlackBore Chokes help you shoot your best, no matter if you have a standard barrel or backbored barrel! By consistently slowing the wad at the moment the shot leaves the barrel/choke, the patterns from your shotgun will always be dense and evenly distributed because the wad will not "blow thru" the shot column as the shot exits the barrel!