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Layne Simpson's explanation...

Also called overboring, backboring simply means that the bore diameter of a barrel exceeds what has long been the industry standard. Depending on whose chart you believe, .729 to .730 inch is standard for the 12 gauge while the bore diameter of a backbored barrel usually measures from .735 to .740 inch and sometimes a bit larger. As for any major benefit, some shooters are convinced that backboring along with lengthening of the forcing cone of a barrel reduces recoil, and while I don’t believe this has been proven to be true scientifically, I am sold on the concept. The idea of backboring a shotgun barrel is a very old one, but it did not prove to be entirely practical until the introduction of the modern plastic wad with a flanged overpowder cup capable of obturating sufficiently to seal off the oversized bore.
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http://hunting.about.com/od/shotgun/l/aastshotgunfaqa.htm

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http://www.guncustomizing.com/tech.htm#backbore
 

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quickdtoo said:
Layne Simpson's explanation...

Also called overboring, backboring simply means that the bore diameter of a barrel exceeds what has long been the industry standard. Depending on whose chart you believe, .729 to .730 inch is standard for the 12 gauge while the bore diameter of a backbored barrel usually measures from .735 to .740 inch and sometimes a bit larger. As for any major benefit, some shooters are convinced that backboring along with lengthening of the forcing cone of a barrel reduces recoil, and while I don’t believe this has been proven to be true scientifically, I am sold on the concept. The idea of backboring a shotgun barrel is a very old one, but it did not prove to be entirely practical until the introduction of the modern plastic wad with a flanged overpowder cup capable of obturating sufficiently to seal off the oversized bore.
[/color]
http://hunting.about.com/od/shotgun/l/aastshotgunfaqa.htm

more...

http://www.guncustomizing.com/tech.htm#backbore
Additionally my understanding is that it improves patterning due to less shot deformation.
Downside to it is that you should not shoot slugs in an overbored barrel -- the slugs are not able to obturate the oversized bore resulting in poor accuracy.
 

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Hi All,

Now this really makes me laugh, if a shotgun is bored to 0.740" this it's no longer a 12 bore it's probably an 11 bore. The bores of a shotgun reflect the number of lead balls it takes to make 1lb in weight and that fit snugly in the bore. Now if you fire a 9mm bullet down an 38 special bore you will get less felt recoil as the firction and so pressure is less. Put a chamber adaptor in a 10 bore shotgun and fire a 12 bore catrridge and you will also get less recoil but you will also get less velocity. Nothing strange about that. It's a neat marketing ploy though :D.
 
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