Here's the second section on the Tom Roster article in Sporting Clays Magazine. Remember, these are regarding his work and not mine:
"Over-boring, or back boring, is currently also a popular fad in shotgunning. It's hard to buy many specialized models of target shotguns and even turkey or waterfowl guns that do not feature over-bored barrels.
"The burning question is, 'Does over-boring really do anything, and, if so, what should the dimension be?' To find out, I had 12-ga barrels over-bored in .005" increments from a starting dimension of .730", then measured the effects on patterns and velocity.
"I found that out to a finished dimension of no greater than .745" (.015 increase), I measured improvedment in pattern density by as much as 10 - 15% at 50 yards with lead shot loads, especially in shot No. 5 and larger. For shot sizes 6 through 8, I commonly measured an improvement of about 7% in pattern density at 40 yards. With steel shot loads in all shot sizes tested (6s through BBs), I measured an across-the-board increase in pattern density of about 5% at 50 yards and as much as 10% at 60 yards. In sum, there is a small (but to some shooters important) improvement in pattern density as a result of over-boring 12-ga barrels no larger than .015 from standard.
"Over-bores greater than .745" in 12 gauge were found to also produce improvement in patterns with both lead and steel shot types. However, and this is a big however, after .745", I started to measure increasing velocity loss for both the lead and steel loads tested in such 'radically' back-bored barrels. The velocity loss was greatest with the steel shot loads, due, no doubt, to the lessened ability of the typical stiff steel shot wad's obturating skirt to expand large enough to seal the bore properly (contain powder gasses) as the over-bore exceeded .745".
The Lesson? Back-boring is good for pattern performance up to a point but then becomes a negative, especially in terms of velocity loss. My findings indicate that a 12-ga. barrel over-bored to a finished diameter larger than .745" is a minus, if not a downright mistake, and .740 is better yet."