Sounds like its working to me! Although I'm interested in seeing this article you mentioned to understand the logic and physics behind the reasons, I'm more liable to believe what I have personally experienced first hand. My Browning has a .742" backbored barrel and I noticed marginally better shot patterns over some of my other 12g guns that have the nominal .729" bore using the same choke constrictions. The Mossberg 835/935 has a .775" overbored barrel which is the same nominal bore diameter of a 10g. And as mentioned, shot patterns continued to improve with the oversized diameter....again, using the same choke constrictions.
Here is one write up I kept on file. I will find the other the next rainy day. Have to make hay when the sun is shining especially this year
Cheers
I wish it were that simple. First there is no way to increase "center mass" of a shotshell charge. That's a mathematical fact, and the goal of doing so is sort of the fool's goal of the shotgun world.
As far as the benefits of backboring, for every person that thinks that it is the cat's meow, there is another that thinks it is total bunkum--
Here is what Bruce Buck, a noted shotgun writer on technical matters said in 1995:
4) Backboring: The nominal interior diameter of a 12 gauge
shotgun barrel is .729", but your barrel could measure anything from
.720" to .800" and still handle a 12 gauge shell. Anything bigger than
.729" is technically overbore or backbored. Stan Baker, Seattle
gunsmith, claims to have coined the term "backbore" in order to avoid
the less attractive connotation of "overbore". They mean the same thing.
The Technoid, in one of his denser moments, had some guns
backbored in an attempt to reduce recoil. It did not work and should not
have come as a surprise. If you look at the formula for free recoil, bore
size is not one of the components. From a purely subjective (not
mathematical) point of view, it also failed to lower the recoil sensation
the way that elongating the cones did. Don't know why.
In theory, backboring decreases friction and provides a larger
wad base for the powder gases to push upon. Both of these should
increase velocity and they may to a slight extent. Stan Baker claimed
that his extreme and maximum .800" Big Bore barrels added 50 fps.
Even so, normal shell to shell factory variation is 30 fps, so the most
extreme backboring does not really affect velocity for all practical
purposes.
Very often when people have guns backbored, they also run the
cones out and may also have the gun ported. When everything is done
at once it is impossible to tell if one particular change had a measurable
effect. Subjective recoil reduction caused by elongated cones is often
attributed to backboring or porting.
Instead of decreasing recoil, aftermarket backboring actually
increases it because the weight of the gun is reduced by the amount of
metal removed and ejecta velocity may be increased slightly. Gun
weight and ejecta velocity are important components of the recoil
formula.
There is a big difference between "aftermarket" backboring and
"factory" backbored new guns. The factory backbored barrels are
actually a little heavier because wall thickness is maintained while barrel
diameter is increased. There is more metal. If you want an example of
road hugging weight brought on by factory "backboring", try to swing a
new Browning 425 Ultra with 32" barrels.
Large bores may help slightly when using extremely heavy
hunting loads, but there is no proven meaningful change in velocity or
recoil with standard target loads. The aforementioned Australian tests
found that backboring neither meaningfully increased velocity nor
consistently improved patterns. Sorry folks, factory "backboring" is just
another marketing ploy to go along with barrel porting.
Does aftermarket (not factory) backboring have any benefit at all?
You can bet your Junior Technoid magic slide rule ring that it does.
While aftermarket backboring may not reduce recoil or improve patterns,
it sure reduces weight. If the barrels on your gun feel too heavy and
unresponsive, you may be able to put them on a diet. Most standard
barrels have a wall thickness of around .040". This is a lot of meat and
might be substantially reduced. On a standard 30" set of barrels,
backboring .010" will reduce barrel weight by 2.77 ounces. A .020
backbore will take off a monumental 5.58 ounces. A change of 3 ounces
is a lot, so go easy. Check first with your gunsmith. He will know
what is safe. Be aware, however, that aftermarket backboring will void
any factory warrantee. Briley charges about $150 per tube for
backboring.
Good news/bad news. Backboring works well to reduce the
weight of solid choke barrels. Unfortunately, solid choked barrels are
usually pretty well balanced and seldom need it. It is the factory screw
choke barrels that are usually too loady up front, especially the 32" jobs.
A backbore of 3 to 4 ounces could transform these guns from pigs to
peacocks. That is the good news. The bad news is that Briley does not
want to hear about backboring a gun with factory screw chokes. Now
you know how Tantalus felt when the Greek gods kept the water and
grapes just out of reach. The problem is that enlargement of the bore
may cause the skirt of the unaltered choke tube to intrude into the bore
itself. This would cause the choke to be added to the ejecta on the first
shot! Bad move.
Though Briley will not touch your screw choked Beretta or 425,
Ken Eyster and several other custom gunsmiths may still be willing to
backbore a screw choked gun. Whatever you do, make sure your
backboring is done by a pro
.
Randy Wakeman, "Why Backboring does not Work",
http://www.chuckhawks.com/backboring_does_not_work.htm
Backboring (overbore and underbore barrels) have been experimented with for a long time. It was around 75 years ago that the 12 gauge "3 inch magnum" shell was developed. Shotgun barrel bores were intentionally opened up to the British maximum of .750 in. at that time, with the press back in the day bragging of the lower recoil and better patterning that was observed.
They had it right, but only half right. Anytime a shotgun barrel is increased in inside diameter with the exact same shotshell, both pressure and velocity are reduced due to the increased barrel volume. There is less recoil, but only because muzzle velocity drops.
Pattern densities may often improve, but again due primarily to lower velocities that deform less lead shot on initial setback. You can accomplish the same in any shotgun by lowering the muzzle velocity, all other things remaining constant.
That why touting "backboring" remains one of the pet rocks of shotgunning today. The real reason to backbore is to pull weight out of a heavy set of barrels. Any other significant "benefits" were disproved well over half a century ago.
More highly technical discussion:
http://www.shotgunworld.com/bbs/viewtopic.php?t=130741&postdays=0&postorder=asc&start=15
At the end of the day, you find that the only true money shooters in the shotgun world, live pigeon shooters, do not have their guns backbored or overbored. These are guys who would gladly sell a testicle for one more (real live) bird.
Most to the top trapshooters do not have their guns backbored. These are the second tier money shooters.
and Yes any semi is better than any pump in the recoil department. Next to to impossible patterns that is a funny one
