Back-Boring the Shotgun Barrel

Simply defined, back-boring is the enlarging (by reaming) of the barrel’s diameter from the front of the chamber forcing cone to the choke’s forcing cone. This is normally accomplished on the lathe using a reamer with a tapered leading edge to form or blend into the choke forcing cone. The reamer is driven by along handle’as much as 40 inches long’attached to the lathe spindle. Back-boring can also be done with hand-powered equipment. This is not anew 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, the technique resurfaced. The A. H. Fox Company in Philadelphia had a shotgun-bore mechanic named Burt Becker, whose experimentation with back-boring produced some of the best long-range waterfowl shotguns of their time.

These were the Super Foxes and the HE-Grade Fox shotguns, with bores as much as .014 inch oversized. Perhaps it’s the common cycle of the back-boring trend, or maybe just clever marketing by modern shotgun manufacturers, but today’s shotgunners seem to be sold on the benefits of back-boring in certain sporting applications. It’s possible, I suppose, that back-boring has finally found a permanent place on our list of optional features for the shotgun bore.

Both Remington and Browning now offer over-bored barrels in their target guns. This may fool some gun owners into thinking their trap and sporting-clays guns are obsolete, and indeed many will trade in perfectly good and serviceable guns and barrels to obtain this "new" feature. Other gun owners will simply look for a gunsmith who can back-bore the barrels on their favorite shotguns.

In fact, the shotgun-bore mechanic who can perform this service for today’s target shooter and sportsman will find a rapidly increasing awareness of the benefits of back-boring among his customers’as well as an expanding market for this service. But there are other reasons why the gunsmith should be familiar with this technique. One of the more useful applications of back-boring is in shotgun restoration.

Many older shotguns’especially older side-by-side models’have been subjected to corrosive propellants, corrosive primers, and years of neglect that have left their bores rough and pitted. In the repair or restoration of these older shotguns, anew bore surface is always a good idea. In much the same way an engine mechanic will overbore the cylinders of an internal-combustion engine for a better piston-ring seal, the bore mechanic can renew a pitted bore surface, improving its appearance and wad seal.

When back-boring for restoration, it’s important to adjust the choke diameters as well. Remember that the choke constriction is the difference between the bore diameter and the exit diameter. Increasing the bore diameter has the same effect as closing the choke, and after the bore is restored by back-boring, an equal amount of material must be removed from the choke if the final constriction is to remain as it was originally.

Many older shotguns have been sawed off for one reason or another at some time in their history. Back-boring can give these otherwise cylinderbore barrels some degree of choke. Just stop the reamer about 2 inches from the muzzle, and you’ll have a

constricted area at the end of the barrel. You may have to adjust the choke constriction by your choice of reamer size. With all older shotguns brought infor restoration, be sure to measure the chambers before starting any work on the barrels. 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 completed, and the choke adjusted, you have given the inside of your customer’s barrel a completely new surface from forcing cone to muzzle.

This is a service your customer is sure to appreciate. The back-bore reamers and handles I use are made by Clymer Manufacturing and Loon Lake Precision. This is not in any way an endorsement, more a point of reference, as I will be both picturing them and referring to techniques in their care and use. They are of very high quality, but surely the reamers and handles made by your favorite tool maker will do the job and can be easily adapted to these instructions.

The Clymer/Loon Lake 12-gauge tool handle is 5/8 inch thick (.625). 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 use an adjustable reamer (the same one you use for 20-gauge choke adjustments) to open its bore to .626 inch and cut off a 3-inch section. They need not be expensive; these are made by Skeet’s Gun Shop.

Now measure the outside diameter of this piece (it should be .725 inch or so) and find or size apiece of 12-gauge barrel to fit over it. (For this, you’ll need your 12-gauge choke-adjusting reamer.) This piece should have the same .001-inch clearance over the 20gauge piece as the 20-gauge piece has over the tool handle. Now turn the outside of the 12gauge barrel to .795 inch, and save a 3inch section. Clean and oil your chamber-guide pieces and slide them down the tool handle.

This type of handle guide’with the pieces well oiled’ will ensure a good bearing surface as the tool handle turns and will prevent scoring of both the tool handle and the chamber. Back-boring is an operation best performed on the lathe, as a constant feed rate is critical to a good finish, and a good finish is critical to shotgun-bore performance. Nevertheless, there are gunsmiths who do this operation with power hand tools, and I must confess I’ve done some barrels in this way myself.

One reason for using hand tools is simply for the These were made by gunmaker John Britt. The wooden knob must turn freely. practice, and I’ve used hand tools on a couple of bolt-action shotguns that were too bulky to be done in any other way. So, although I’d prefer to see this done on a lathe, I will include some instructions on back-boring using a

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1/2-horsepower hand drill as a power source. Set up your practice barrel (I urge you not to skip this step) in a wellpadded vise. (Two pieces of 2×4, well grooved from constant use, work very nicely.) If increasing the length of the forcing cone is part of the job, this should have already been done.

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 will have to turn 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 apiece 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 don’t exceed about 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, re-mark the tool handle, and make another cut. Under no circumstances should the tool be allowed to cut 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 certainly result in some degree of distortion.

When the tapered tip of the backbore reamer has just started to blend into the fixed choke, or when you’re within 1/4 inch of a screw-in choke seat, you’re done. Clean the bore and hold it up to a light for inspection. What you want to see is a reasonably smooth bore finish, with no reamer gouges or chatter marks. If you’ve gouged it, the damaged sections will require excessive polishing, and 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. Although it can be done, using a drill motor to power the reamer is more a matter of "feel" than anything else, so my written instruction can only help so much. Beyond the instructions in the last few paragraphs, only experience and practice can produce an acceptable job. Back-boring can also be done without power tools.

A bit of practice and some time to develop a good sense of "feel" will be needed, but a crank handle can be easily made (see photo) and an excellent finish can be had by careful hand rotation of the boring equipment. You’ll be surprised how quickly a back-boring reamer progresses down the bore with light pressure and hand power. Cutting no more than 3 inches at a time and withdrawing the tool to clean chips and re-oil the bore will result in a smooth bore surface that will require very little polishing.

The flutes on the Clymer or Loon Lake back-bore reamer have a lefthand spiral, and there are eight of them for stability. The recommended depth of cut is from .005 to .020 inch. Optimum feed rate will vary between .008 and .016 inch per revolution (depending on the depth of cut). The reamers are designed to rotate at a maximum speed of 75 rpm. As you can plainly see, the best 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. If, like so many gunsmiths, you have a smaller machine, perhaps with no provision for coolant recirculation, you can still perform this service very well. Next I’ll give some instructions for back-boring with these smaller machines without a coolant system.

If you’re fortunate enough to have a large lathe, you’ll quickly realize the advantages of the larger equipment and be able to skip many timeconsuming steps. You’ll need a tool-post-mounted milling fixture. An adequate device is made by Palmgren, and can be found in the Brownells catalog at an extremely modest price, considering the versatility of this tool. Mount the milling vise on your tool post.

The instructions say to clamp it down with a tool-post ring, but I’ve found that an upside-down tool holder secures it nicely. You can hold your back-bor-

ing tool handle either with a 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 can 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.

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 (Brownells Do-Drill or dark cutting oil) in the first 4 inches of the bore. Guide the chamber over the reamer until the forcing cone and reamer touch. Then push the chamber guide into the chamber. The tool and barrel assembly will now stay unsupported.

If your lathe has along enough bed, the muzzle can be held in position with the tailstock center, and the milling vise’s lower jaw can be raised to meet the barrel. Once the barrel is set up, the tailstock is removed. If you’re using a lathe with a shorter bed, you’ll need to "find" the muzzle’s center point. 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 vise, and loosen and 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 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 .729 inch and you’re boring to .740 inch, you’re making an .011-inch cut. This puts you right in the middle of the recommended feed rates.

I use a feed rate of .014 inch per revolution, as I always like to be sure the flutes are adequately loaded to prevent chatter. Note a spot on your ways about 4 inches from your apron and mark it with your finger. This is how far you will go with each cut. Turn on the lathe and engage the drive. When you have cut to the mark on your ways, release the drive and back out the tool under power. But don’t remove the barrel from the milling vise!

Clean out the chips by pushing a patch through the barrel from the muzzle, and re-oil. Brush the reamer clean and then wipe it with a shop rag. During this operation, you’ll have to loosen the collet or chuck and extend the tool several times. As your tool approaches the muzzle, observe its position with a flashlight. You’ll 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. I like to stop 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 clean and properly oiled during the operation, there are no gouges. If the reamer was properly loaded, you should find no chatter marks. And if you were careful in mounting the barrel in the milling vise, you haven’t left a single mark on the barrel’s bluing.

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 crucial to shotgun bore and choke performance. If you don’t believe me, take the practice barrel you just back-bored out to the range and pattern it before you begin the polishing process. Then pattern it between each successive polishing step.

What you’re going to find is that choke performance and pattern density improve with each successively finer polishing operation. That’s because the better the surface of the bore is polished, the less disruption there is to the protective shotcup. There are a number of flexible honing systems and oils on the market, but I’ve found none that work as well as this homemade fixture. To make one that fits 16- and 12-gauge bores, go to your local hardware store and buy several 48-inch pieces of 5/8inch hardwood dowel.

For 20-gauge bores, I recommend apiece of 1/2inch aluminum rod, as a 1/2-inch hardwood dowel will not have adequate strength. You will also need emery cloth in coarse, medium, and

fine grits, as well as Scotch-Brite pads in both medium and fine grit (green and gray). Hold apiece of dowel or aluminum rod vertically in your bench vise and slit it down the center with a hacksaw (about 3 inches). Remove your milling attachment and crank the apron to the left, well out of the way. Lock the dowel in your three-jaw chuck, leaving enough dowel exposed to polish the whole barrel. Disengage the back-gear and lock the spindle. Adjust the speed. You’ll want to polish at a relatively high speed.

Now take apiece of coarse emery cloth (often you can start with medium) about 3 inches wide by about 8 inches long, and insert one end in the dowel slit and wrap the rest clockwise around the dowel (as you face the spindle). You’ll have to experiment with the length of the emery cloth. You want it to fit relatively tightly in the bore, but if it’s too tight, the hardwood dowel will fail. Wear safety glasses when doing this, as an exploding dowel can send splinters all over the place.

There will be a lot of heat generated by polishing, so a pair of leather work gloves is also a good idea. Spray penetrating oil down the bore, place the chamber over the end of the fixture, hold on tightly, and turn the power on. Polish, slowly and evenly, from the forcing cone to the throat of the choke. The barrel will get very hot during this process. Once 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 unused end that had been inserted 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 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 look very good now, probably better than many factory barrels. Re-oil the barrel and repeat the polishing process with medium and then fine emery cloth. The medium and fine grits should require much less polishing than the coarse cloth did, since you are only polishing to remove the sanding marks left by the previous emery cloth. 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’re finished polishing with the fine cloth, you’ll have an excellent finish. As good as your polished bore finish is, it can be improved a bit by further polishing with Scotch-Brite pads. You’ll need to spread the slit in the dowel a bit with a small knife or screwdriver to install the Scotch-Brite and wrap it 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 end to end, dry, first moving it slowly and then rapidly. This will produce across-hatch pattern in the polished surface. This type of polishing pattern will be the least disruptive to the protective shotcup, and will soon disappear with a little use. A medium (green) pad will give you a beautiful bore surface, but you may choose to follow this up with a fine (gray) pad.

Another method of producing a good final finish is dry polishing 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-boring job will polish up in about 10 minutes. One other note: If your customer is interested in having screw-in chokes installed in his barrel, this service will have to be done before backboring.

This is the best way to ensure a good pilot fit and to avoid exceeding the maximum bore diameter at the choke seat. What you’ve just produced is a barrel bored to your customer’s exact specifications, with a mirror finish inside from end to end. There are not many shotgun manufacturers who take this care in producing their product. One look down the bore of a Remington, Ithaca, or Winchester will prove this. You could even offer a polishing service alone.

Your customers who own shotguns with factory barrels will find that polishing will improve their shotguns’ performance even without back-boring. I