February 19, 2014

Back-Boring Shotgun Barrels

Borrowing a little technology from the 19th century will benefit some of your clay-target shotguns.

In its simplest definition, back-boring is the opening of the barrel diameter from the front of the chamber to the back of the choke. This is normally accomplished on the lathe using a reamer whose leading edge is tapered to form the choke forcing cone, and is driven by a long handle (as much as 40 inches) attached to the lathe spindle. This is no new practice. British gunmakers started using this technique in the late 19th century. Back then they called it freeing (not to be confused with free-boring), but this practice soon lost favor in light of the development and refinement of the fixed choke.

Early in the 20th century this trend resurfaced. The A.H. Fox Company in Philadelphia had a shotgun-bore mechanic named Burt Becker. His experimentation with back-boring produced some of the best long-range waterfowl shotguns of their time. These were the Super Foxes and HE-grade shotguns, and their bores were as much as .014 inch oversized.

The modern-day shotgunner is now waking up to the benefits of back-boring in certain sporting applications. This may be just the common cycle of the trend, or perhaps this feature has finally found a permanent place in our list of optional features for the shotgun bore. Both Remington and Browning now offer this feature in their target guns. This may fool some gun owners into thinking their trap guns are obsolete; indeed, many will trade in perfectly good and serviceable guns and barrels to obtain this “new” feature.

To perform this service properly, it is important that you understand what back-boring does and how it affects shotgun performance.

What Back-Boring Is

Back-boring lowers barrel pressure, shortens the shot string, and increases the effectiveness of the fixed or screw-in choke. To understand how back-boring does these things, we must accept some basic rules. Notice that I refer to rules, not laws. Rules have exceptions, laws do not. Anytime you increase the size of a bore, or for that matter a garden hose, you increase its volume and lower its pressure.

Lowering bore pressure in this manner will benefit the shotgunner in most cases. Back-boring shortens the shot string. By increasing the size of the bore, you will widen and therefore shorten the length of the shot column. This will result in more pellets reaching the intended target at the same time. The trap shooter and hunter alike could benefit. But on the negative side, while back-boring a slug barrel will certainly lower its internal pressure, it will also cause a loss of accuracy by allowing the slug to fit poorly within the bore.

Generally speaking, back-boring increases the effectiveness of the choke. We all understand the choke’s effectiveness is in part determined by the amount of constriction, not the size of the muzzle, as those who measure such things with a dime would have us believe. Changing the size of the bore without taking the exit diameter into account might easily have a detrimental effect on choke performance. If you increase the size of a bore on a shotgun that already patterns extra-full and leave the choke alone, you might very well ruin that barrel’s performance by causing an overconstriction of your customer’s favorite load. By the same logic, back-boring a customer’s favorite improved cylinder grouse gun might easily turn it into a modified choked gun and cause him to miss a lot of close shots he might otherwise have made.

Common sense and pattern testing will enable you to advise your gun owner how he might best benefit from back-boring. Remember, even if you do a great job of back-boring a gun and return a mirror-finished, perfectly straight bore, if the choke no longer functions the way he needs it to, he might easily think your back-boring service has ruined his gun. Many people will not give you a chance to correct such things. It seems some people would rather have something bad to say about others than give them a chance to make things right. Since back-boring affects choke performance, be sure to explain this to every owner and offer to make any necessary adjustments in choke that might be needed in the future.

American Gunsmith, Book of the Shotgun. Chapter 15

Courtesy, American Gunsmith

In the back-boring set-up on the lathe, the toolpost-mounted milling fixture holds the chamber end of the shotgun barrel, while the reamer’s handle is attached to the lathe headstock.

Many older shotguns have been sawed off for one reason or another sometime in their history. Back-boring can give these otherwise cylinder-bore barrels some degree of choke. Just stop the reamer about 2 inches from the muzzle and you will have a constricted area at the end of the barrel. You may have to adjust the choke constriction by your choice of reamer size.

In the restoration of older shotguns, back-boring can make a badly pitted bore as good or better than new. When back-boring to restore an old bore, you will probably want to make a less extreme increase in bore diameter. These will usually be bored to 0.730-inch or 0.735-inch in 12 gauge and similar minimal increases in the smaller gauges. Again, remember you are increasing choke constriction when you increase the size of the bore, and that many of the older shotguns already had very tight chokes because they were designed before the advent of the protective shot cup. These will almost definitely need to have their chokes opened a bit. This gives you a chance to restore not only the bore but also to give the parallel section of the choke a new surface. Measure the chambers on all older shotguns brought in for restoration. Many of them are short by today’s standards and will have to be lengthened to 2 3/4 inches.

With the chamber lengthened, back-boring complete, and choke adjusted, you have given the inside of your barrel a completely new surface, from forcing cone to muzzle. Now this older shotgun has been brought up to today’s ammunition standards, both in chamber length and choke constriction. This is how you do the work:

How to Back-Bore

The back-bore reamers and handles I use are made by Clymer Manufacturing. This is not in any way an endorsement of these tools; I mention them more as a point of reference because I will be both picturing them and referring to techniques in their care and use. Nonetheless, the Clymer tools are of very high quality, but certainly reamers and handles made by your favorite tool maker are acceptable.

The Clymer tool handle is 5/8 inch thick. The first accessory you’ll need to operate it is some sort of guide to hold the handle centered in the chamber. In 12 gauge, this can be made very easily. Take an old piece of 20-gauge barrel and, with an adjustable reamer (the same one you use for 20-gauge choke adjusting), open its bore to 0.626 inch and cut off a 3-inch section. Now measure the outside diameter of this piece (it should be 0.725 inch or so) and find or size a piece of 12-gauge barrel to fit over it (for this you will need your 12-gauge choke-adjusting reamer). This piece should have the same 0.001 inch clearance over the 20-gauge piece as the 20-gauge has over the tool handle.

Turn the outside of the 12-gauge barrel to 0.795 inch and save a 3-inch section. Clean and oil your chamber guide pieces and slide them down the tool handle. Using this type of handle guide and keeping the pieces oiled will ensure a good bearing surface as the tool handle turns and will prevent scoring of both the tool handle and the chamber.

This is an operation best performed on the lathe as constant feed rate is critical to a good finish and a good finish is critical to shotgun bore performance. There are those among us who will attempt this operation with power hand tools—I confess that I have done some in this way; one for practice and a couple of bolt-action shotguns that were too bulky to be done in any other way—so I will include minimal instruction on back-boring using a 1/2-horsepower hand drill for power.

American Gunsmith, Book of the Shotgun. Chapter 15

Courtesy, American Gunsmith

Once the back-boring is completed, the gunsmith will need to adjust the choke to match the shotgun’s new bore diameter. For this job, a hand-held adjustable reamer (top) is the best choice, followed up with a professional polishing (bottom).

Set up your practice barrel (I urge you not to skip this step) in a well-padded vise (two blocks of 2-by-4 wood, well-grooved by constant use). If increasing the length of the forcing cone is part of this job, do that first.. This will enable your chamber guide to start a little more deeply within the chamber. Unless you have a 5/8-inch chuck on your drill, you have already turned the last inch of your tool handle down to 1/2 inch.

Squirt a good amount of high quality cutting oil (four or five squirts from your oil can should be enough) toward the first 4 inches of bore. Insert the back-bore reamer but apply no inward pressure at this time. Slide your chamber guide into position. Now it’s time to mark your tool handle. Put a piece of masking tape on it about 4 inches from the rim cut. This is as much as you’ll be able to cut at a time without risking your reamer becoming overloaded with chips and gouging the bore. Start your drill but do not exceed 75 rpm, and hold tightly while applying firm yet even inward pressure. When you’ve cut 4 inches, withdraw the tool under power until the back of the reamer contacts the chamber guide. Clean and re-oil the bore, clean the reamer, and wipe it with a shop rag, remark the tool handle and make another cut. Under no circumstances should the tool be allowed into the section of barrel within the vise jaws. There is a certain amount of “crush” at this point and running the reamer to this position will result in disaster.

When the tapered tip of the back-bore reamer has just started to blend into the fixed choke, or you are within 1/4 inch of a screw-in choke seat, you are done. Clean the bore and hold it up to a good light for inspection. You want a reasonably smooth bore finish with no reamer gouges or chatter marks which would require excessive polishing. Your chances of producing a consistent diameter throughout the bore after polishing are slim. If you need more practice, grab another old barrel and try again. Using a drill to power the reamer is such a matter of feel that any degree of instruction will only help a little. Only when you have produced a reasonable finish on a scrap barrel, and you’re sure you can repeat it, are you ready to back-bore a customer’s gun. Your barrel is now ready for polish.

The flutes on the Clymer back-bore ream­er have a left-hand spiral and there are eight of them for stability. The recommended depth of cut is from .005 inch to .020 inch. Optimum feed rate will vary between .008 and .016 inch per revolution (depending on depth of cut). The reamers are designed to rotate at a maximum speed of 75 rpm. As you can plainly see, the only way to maintain these parameters accurately is by back-boring on the lathe.

If your lathe apron has a travel of over 30 inches and you have a coolant recirculating system, you have it made and can skip many time-consuming steps. If your machine is smaller and doesn’t have a provision for coolant recirculation, you can still perform this service very well.

You will need a tool post-mounted milling fixture. An adequate yet versatile and inexpensive one is made by Palmgren and is available through Brownells. Mount the milling vise on your tool post. The instructions say to clamp it down with a tool post ring, but an upside-down tool holder will do just as well. You can hold your back-boring tool handle either by 5/8-inch collet or in a three-jaw self-centering chuck. In the initial setup, remember my warning about letting the tool enter the area of the bore being “crushed” by the vise. This will be avoided by mounting the barrel at the chamber. Again, if lengthening the forcing cone is part of this job; do it first. The more of the chamber guide bearing surface actually engaged, the truer your setup will be.

American Gunsmith, Book of the Shotgun. Chapter 15

Courtesy, American Gunsmith

A hardwood dowel wrapped in oiled emery cloth polishes the shotgun’s bore.

Put your tool handle through the headstock, allowing the tool to protrude about 12 inches and tighten the collet or chuck for now. Squirt plenty of high-quality cutting oil into the first 4 inches of the bore. Guide the chamber over the reamer until the forcing cone and reamer touch. Push the chamber guide into the chamber.

The tool and barrel assembly will now stay unsupported. Grab the muzzle and get a feel for how much play there is in your unsupported barrel then hold the barrel in the center of that play. Raise the lower jaw of the milling fixture to meet the barrel. Now tighten down the milling vice and loosen, then retighten the bolt controlling the tilt of the vise. This will correct the “tilt” and align your bore to the secured tool. Check your setup with a good level on the outside of the barrel. Engage your back-gear and set your lathe to run below 75 rpm; I like to run mine at 60 rpm. Set your feed-rate. If you have a bore of 0.729 inch and are boring to 0.740 inch you are making an 0.011 inch cut. This puts you right in the middle of the recommended feed rates. I would choose 0.014 inch per revolution feed rate to be sure the flutes are adequately loaded to prevent chatter. Note a spot on your ways 4 four inches from your apron and mark it with your finger. This is how far you will go with each cut. Turn the lathe on and engage the drive. When you have cut to the mark on your ways, release the drive and back out the tool under power. Do not remove the barrel from the milling vice! Clean out the chips by pushing a patch through the barrel from the muzzle and re-oil. Brush the reamer clean, then wipe it with a shop rag. During this operation you will have to loosen the collet or chuck and extend the tool many times.

As your tool approaches the muzzle, observe its position with a flashlight. You will want to stop the feed when the tapered front end of the tool just begins to blend into the existing choke. If the barrel has screw in chokes, be careful to stop the reamer before it reaches the choke seat, about 1/4 inch before the seat.

If all has gone well, you now have a nicely back-bored barrel. If you kept the reamer properly oiled and clean during the operation, there are no gouges. If the reamer was properly loaded, you will find no chatter marks. If you were careful in mounting the barrel in the milling vise, you have not left a single mark on your customer’s blueing. You may see evidence of where you started and stopped the operation on the walls of the bore, but these are very shallow imperfections and will disappear in the polishing process.


A well-polished finish is critical to shotgun bore and choke performance. If you don’t believe me, pattern the practice barrel you just back-bored before you begin the polishing process and again between each successive polishing step. What you’re going to find is that choke performance and pattern density improve with each successively finer polishing medium. This is because the protective shot cup is disrupted less as the surface of the bore becomes more polished.

There are a number of flexible honing systems and oils on the market, but I have found none that work as well as this homemade fixture. To make one of your own, go to your local hardware store and buy several 48-inch pieces of 5/8-inch hardwood dowel and emery cloth in coarse, medium, and fine grits. You will also need a cleaning rod, bronze brush, and some fine Scotch-Brite pads.

Hold a piece of dowel vertically in your bench vise and slit it about 3 inches down the center with a hacksaw. Remove your milling attachment and crank the apron to the left, well out of the way. Lock your dowel in your three-jaw chuck, leaving enough dowel exposed to polish the whole barrel. Disengage your back-gear and lock the spindle. Adjust the speed. You’ll want to polish at a relatively high speed. Now take a piece of coarse emery cloth (often you can start with medium) about 3 inches wide by about 8 inches long, insert one end in the dowel slit and wrap the rest clockwise around the dowel (as you face the spindle). You will have to experiment with the length of the emery cloth because, while you want it to fit reasonably tightly in the bore, too tight a fit will cause the hardwood dowel to fail. If this should happen, and it occasionally will, be sure to be wearing safety glasses. An exploding dowel will send splinters all over the place. Spray a lot of penetrating oil down the bore, place the chamber over the end of the fixture, hold on tight and turn the power on.

American Gunsmith, Book of the Shotgun. Chapter 15

Courtesy, American Gunsmith

Necessary back-boring implements include, from bottom, a handheld drill with a brake-cylinder ball hone, a dowel and emery cloth, a Palmgren milling vise, two reamers (one with the drive handle and chamber guide attached), bore brush, a handheld reamer, emery cloth, a gallon of cutting oil, and a level.

Polish slowly and evenly, from the forcing cone to the throat of the choke. The barrel will get very hot during this process. After you’ve gone end-to-end twice, turn off the power with the fixture still inside the chamber. Re-oil the barrel and turn the emery cloth around, using the fresh end that had been in the slit, and polish once more. Never turn on the power without the paper end started in the chamber and be sure to turn the power off before removing the barrel completely.

Now that you’ve polished twice with coarse cloth, clean the bore and hold it up to a strong light for inspection. There should be no evidence of the reamer at this point. Actually, the bore will probably look better than many factory barrels, but you’re not finished.

Re-oil the barrel and repeat the polishing process with medium, then fine emery cloth. These grades should require much less polishing time than the coarse cloth did. Be sure to move the barrel evenly across the polishing fixture. You are trying to maintain an even bore diameter throughout. A little practice will give you the feel you need to do this.

When you have finished polishing with the fine cloth, you will have an excellent finish, but don’t stop yet. Remove the handle from your cleaning rod and secure the handle end in the three-jaw chuck. Put a bronze brush on the other end and wrap fine Scotch-Brite around the brush (again, clockwise as you face the spindle).

You want the Scotch-Brite to fit relatively tightly in the bore. Insert the Scotch-Brite in the chamber and turn on the power. Polish the barrel dry from end-to-end, moving it slowly then rapidly. This will produce a crosshatch pattern in the polished surface. This pattern will be the least disruptive to the protective shot cup and will soon disappear with a little use. Another method of producing a good final finish is to dry polish with medium emery cloth fitting very tightly in the bore. Some gunsmiths prefer this method for its speed. As the cloth loads up, it will burnish the surface of the bore. A good back-bore job will polish up in about 10 minutes.

One other note: if you want screw-in chokes installed in the barrel, this will have to be done before back-boring. This is the only way to ensure a good pilot fit and that maximum bore diameter at the choke seat is not exceeded.

What you have just produced is a barrel bored to your exact specifications, with a full-length mirror finish. There are not many shotgun manufacturers who take this care in producing their product. One look down the bore of an over-the-counter Remington, Ithaca or Winchester will prove this. ★

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