Back-bore refers to increasing the bore diameter of a shotgun greater than the nominal gauge / bore specification but not to exceed the SAAMI maximum.
Some of the reasons that you might want to back-bore a shotgun are:
- Increased shot velocity. Increasing the diameter of the bore decreases friction inside the barrel thus increasing shot velocity.
- A more uniform and effective shot pattern. Increasing the diameter of the bore decreases the internal pressure allowing for fewer deformed shot and a more uniform and effective shot pattern.
- A reduction in perceived recoil. Increasing the diameter of the bore decreases the internal pressure allowing for an effortless transition of shot through the barrel thus reducing the perceived recoil and increasing shooter comfort.
- Adding some ‘choke’ to a shorted barrel with thin walls. Shortened barrels that are too thin to be threaded for screw in tubes may be able to be back-bored to within 2″ / 3″ of muzzle to provide some ‘choke’. The resulting degree of ‘choke’ is dependent on the difference in diameter between the original bore at the new muzzle and the diameter of the new bore as well as the lead angle of the reamer.
- Restoring the bore surface. Full-length back boring can restore some or all of the interior surface of rusted and/or pitted bores.
For example, 12 gauge shotguns can be back-bored from .725″ to .745″ in .005″ increments. Thus the resulting bore diameter can be .725″, .730″, .735″, .740″ or .745.”
Footnotes / Sources:
“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.