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The backboring that Vang does probably has more affect on lowering shot charge speeds than the porting alone, but together they do slightly reduce velocities.
The combination of porting, backboring, and forcing cone lengthening all combine to reduce recoil, too.
Just curious as to why you think that backboring reduces velocity?
My area of interest lies in sporting clays vs defensive shotgunning, however I've run tests that show just the opposite with target and hunting loads.
I took a set of 34" Perazzi barrels that were aready overbored from the factory @ 18.7 mm (.736") and had Briley lengthen the forcing cone and backbore to .745" and ream the chokes to the original configuration (23 and 32=tight Mod and slightly tight Full).
Velocity of 7/8 oz Rio target loads increased by an average of 37 fps. Velocity of 1 1/4 oz Kent Fast Lead Pheasant loads increased by an average of 20.3 fps.
In both cases the patterns showed more pellet strikes in the 30" circle at 40 yards.
In both cases muzzle rise (as recorded by time lapsed photo of a lamp mounted on the muzzle) decreased by an average of over 20% with extreme shots exceeding 30%.
And that's in barrels that were not ported.
Full details in December (2009)
Sporting Clays Magazine.I can't imagine a reason that anything would differ with buckshot and/or slugs. Have you chronographed (or seen detailed reports) of before and after velocity results with buckshot/slugs??? I'd be curious.
Greg
