Back Bored.
 A back bored shotgun barrel simply means that the bore has been altered to a larger inside diameter. Backboring of any imparticular gauge does; beyond any doubt, add to more wider center mass and will help reduce the length of shot strings in shotguns and when done within reason it does not decrease the velocity of the load. The reason that it does not decrease the velocity is todays plastic wads will expand a great deal to seal the bore of the shotgun and not allow gas to escape around the wad. NO gas leak, NO velocity lost!

 Back boring means less choke constriction.
 As you open up the bore of a shotgun it takes less choke constriction to still get the same pattern as one with a standard bore. This is a big bonus to all that love to shoot trap and other sporting clay games because this puts more pellets into the center mass of the shot string and over all shortens the shot string reducing the time frame from first pellet strike to the last.

 Choke constriction can be measured somewhat in thousands of an inch plus or minus from the bore diameter to a designated choke size such as 12 gauge bore.729 full-.035 constriction, modified-.019 or imp.cylinder-.009.
 Open up the bore then you open up the choke for the same choke result.

  To study and understand more about shotgun bores.
  Gun Digest Book of Shotgun Gunsmithing By Ralph T. Walker.

 If you do decide to do this.
 Make sure you send your barrel to someone that is experienced with this procedure and their level of finish work is excellent. This is by no means an easy process to get right. Back boring can have a second advantage also if there are defects in the bore of your shotgun then back boring will most likely remove the defect.

 Shotgun Bore Effects - Back bored barrel:

 (1) Will back boring increase velocity? Yes
 A back bored barrel will increase velocity in a 12 gauge reliably up to .740 diameter. All wads will expand to seal the bore up to .740 diameter. The reason you have increases in velocity is that you are changing the volume of the interior of the bore which changes the expansion ratio for the powder being burned. All of this works with the corelation of barrel length but by decreasing the resistance of the bigger bore you will get an increase in velocity.

 (2) Will back boring shorten shotstrings? Yes
 A shorter shotstring will occur for the simple reason there is more center mass because of the larger bore diameter.