Understanding shotgun back bore

What is back boring? Why back bore a shotgun?

What is back boring?

Most shotgunners are familiar with the phrase back boring.  Often it is advertised as a way to reduce recoil and improve shot patterns.  A shotgun with a back bored barrel has its bore opened up to a larger diameter than when it was originally manufactured.

A quick caveat about bore size in shotguns, if you take a few minutes to research back boring, you’ll encounter a definition that sounds something like, “back boring a shotgun barrel is a process by which the bore is opened up over the standard diameter“.  While this definition works, and I’ve used it in my writing, it is an oversimplification, particularly, when referring to the standard diameter of the bore.

custom 870 wingmaster with ghost ring sights and cerakote

So, what’s the standard bore size of a 12 gauge smooth bore shotgun?  It depends on the manufacturer.  Often you’ll hear .729″ thrown around as the standard bore diameter for 12 gauge shotguns in the United States and .725″ for European guns.  While this may be true on guns that you measure, Sporting Arms and Ammunition Manufactures Institute (SAAMI) specifications for a 12 gauge smooth bore barrel are .725″+.020″.  That means a bore that is .740″ is still within “standard” size.  A complete copy of the SAAMI shotshell standards can be found here.  Examining SAAMI standards, notice a +.020″ tolerance in the bore diameter for all smooth bore shotgun barrels- 10, 12, 16, 20, and 28 gauge as well as .410 bore.

Understanding shotgun chokes is part of understanding back boring.  A shotguns choke is how much the muzzle constricts when compared to the bore diameter.  This constriction will correlate to the shotguns pattern, with the tighter constriction typically giving more dense patterns (note: in a select few specialized loads the opposite can occur, but as a rule, the pattern will get tighter) .

A table showing choke constriction for 12 gauge shotguns is shown below.  Note that this table only shows chokes for a 12 gauge barrel, other gauges have different constrictions for a given choke.

shotgun choke constriction

sectioned mossberg 500 barrel w labels

The photo above shows the cross section of a fixed choke 12 gauge Mossberg 500 barrel.  On this sample, the bore diameter was .727″ and the choke diameter .692″.  Using the table above, we can determine the choke of this barrel, .727″-.692″= .035″, is a full choke.

Why back bore a shotgun?

If the Mossberg barrel above was back bored to .740″, it would have a extra full choke (.740″-.692″=.048″).  As you can see in this example, back boring a gun with a fixed choke, is a method used to adjust, or tighten chokes.

Alternatively, a choke can be opened by using an expanding reamer and removing material from the inside diameter of the choke.  In the barrel above, if the muzzle was reamed from .692″ to .710″, the barrel would have a light modified choke (.727″-.710″=.017″).

To measure the bore or choke diameter of a shotgun, Brownells sells bore gauges that allow a precise reading.  These specialized tools use a standard and dial indicator to measure bore diameter to within .001″.

On older shotguns, back boring can be used to recut the surfaces of a damaged or pitted barrel.  In this instance, a reamer slightly larger than the diameter of the barrel would be used to cut a new surface inside the barrel.  Reamer makers, like Manson Precision Reamer, typically offer back bore reamers in .005″ increments.

Some manufactures offer back bored barrels on guns with interchangeable choke tubes.  Often, this is advertised as reducing recoil.

Take a look at this example from Browning (link to source):

“Question:
What is back-boring?

Answer:

Better patterns and increased shot velocity help you hit what you’re shooting at. A back-bored barrel is a shotgun barrel that has a bore diameter increased beyond standard specifications. Increasing the bore diameter of a shotgun barrel greatly enhances its performance. A larger bore diameter reduces friction of the shot charge against the barrel wall. Instead of trying to overcome friction, powder gases expend more energy on the wad base, resulting in an increase in shot velocity. Because there is less constriction or pressure from the barrel walls on a shot charge, there are fewer deformed shot pellets. A greater number of ballistically superior round pellets in a shot charge gives you exceptionally uniform patterns — delivering more shot pellets in the effective part of your pattern.

The effortless transition of a shot charge through a barrel with a larger bore diameter results in a reduction in perceived recoil. Every reduction in recoil results in an increase in shooting comfort.  Therfore (SIC), less felt recoil makes your shooting more enjoyable.”

Beretta does the same thing.  Check out this line from page 3 of their competition shotgun catalog, all Beretta competition shotguns feature back-bored barrels, which greatly reduce felt recoil (link to source).

While the physics suggest lower pressures (as volume increases, pressure decreases), I haven’t ever known this to have an effect.  I personally view back boring as a great way to tighten the pattern of a fixed choke gun, not as a method for reducing recoil.

I don’t mean to pick on Browning or Beretta, I own a couple of their shotguns and love them, but, I don’t understand why a shotgun with an interchangeable choke system should be back bored, nor do I understand how a factory shotgun can be back bored, SAMMI allows for a bore diameter tolerance that is .020″ over!

I have to agree with the following excerpt from an article I found at Chuck Hawks, what factories tout as “back-bored” barrels are not back-bored at all, they are merely over-bored. From what I’ve experienced, this is more of a “placebo effect” than anything else, in addition to being a marketing tool.

Winchester unplated buckshot

Winchester lead 00 buckshot

I had great results improving the performance of an 12 gauge Remington 870 barrel with Winchester Super-X buckshot.  I compared it against a few other barrels and this is how it measured up.

18 barrel patterns buckshot

15 yards: (Clockwise from bottom left) Back bored, Rem Cylinder, Vang Comp, Back Bored 2" from muzzle, Rem Improved Cylinder

15 yards: (Clockwise from bottom left) Back bored, Rem Cylinder, Vang Comp, Back Bored 2″ from muzzle, Rem Improved Cylinder

In closing, back boring is a great way to tighten the choke of your fixed choke shotgun.  On an old shotgun, pitted surfaces can be recut with a back bore reamer.  If your factory gun with interchangeable chokes did or didn’t come back bored, I wouldn’t worry about it.  You most likely can’t tell the difference.

Further reading:

  1. Tactical Shotgun Build: Back Boring a Barrel and Lengthening a Forcing Cone to Increase Performance–   Shows how a barrel is back bored by hand.
  2. Removing a fixed choke from a shotgun barrel and installing a choke tube system  Demonstrates the use of an expandable reamer to remove a fixed choke prior to the installation of an interchange choke tube system.
  3. SAAMI Shotshell standards
  4. Choke Adjusting, from Brownells
  5. Manson Reamers, manufacturers of back bore reamers