December 24, 2013

Adjusting Fixed Chokes In Single-Barrel Guns

Modifying the choke-exit diameters in shotguns with fixed chokes is a common chore. Here are the basics of getting that job done.

The fixed, or “built-in,” choke is found in many of today’s shotgun barrels, and it is found in almost all shotgun barrels over ten years of age. It generally consists of a tapered entry (choke forcing cone) followed by a parallel section. Without a doubt, the most frequent alteration that the shotgun bore mechanic is asked to do is to make an adjustment in a choke exit diameter.

Opening fixed chokes has been made necessary by the evolution of American shotshell ammunition. In the 1950s, a major improvement was made in the shotshell ammunition available to the American shotgunner. The use of the protective shot cup, and later the one-piece wad with integral shot cup, made the fixed choke much more efficient. It was not uncommon for Modified-choke barrels to shoot Full-choke patterns with this newly improved ammunition. European shotgun barrels are still made with extremely tight chokes, since much of the ammunition used in Europe still does not have a protective shot cup. In the 1980s, nontoxic (steel) shot was mandated for use in waterfowl hunting and even for use in upland bird hunting on some federally owned land. Steel requires fixed chokes to be opened even more in order to get usable patterns.

Adjusting the fixed choke is generally done by the use of expandable reamers. This operation can be done by hand, using an adjustable reamer attached to a tap handle, or it can be done on the lathe using a back-piloted reamer attached to a long drive shaft, which in turn is attached to the lathe spindle and operated at approximately 75 rpm. I will discuss how to do both methods, but no matter which method is used, the choke adjusting job starts with an accurate measurement of both the bore and the choke exit diameters.

Diameter Measurement

The fastest way to accurately measure these diameters is by using a dial bore gauge. This is a reasonably expensive device and if you don’t yet have one, a set of inside dividers and a micrometer or a set of screw-in choke installation pilots can also be used. Once you have a bore and a choke measurement, write them down and subtract the choke figure from the bore. If, for example, you are starting with a barrel marked Full choke, you probably have a bore measurement of about .728 inch and a choke diameter of about .690 inch; this would make your Full choke .038 inch. By increasing the size of the choke exit diameter, you can decrease the choke constriction to any size desired, as long as it is less than .038 inch. Let us say, for example, that you would still like to get Full-choke patterns, but will be using steel shot. You will likely want the choke opened to .020 inch. To do this you will want to ream the choke exit diameter to .709 to .710 inch (the last thousandth or so will be removed by polishing).

American Gunsmith, Book of the Shotgun. Chapter 16

Courtesy, American Gunsmith

This is a drawing of a typical fixed shotgun choke. The section that will need to be opened to give larger patterns is the parallel section at the muzzle.

Choke Adjusting By Hand

Place the barrel in a padded vise, with enough barrel exposed to the reamer that it does not travel between the vise jaws (there will be a certain amount of “crush” between the jaws). Adjust your reamer until it just starts to cut as you rotate it clockwise within the bore. Oil the reamer and continue to turn the reamer until you feel it pass through the choke and stop cutting. Back out the reamer, while continuing to rotate it clockwise. Clean the bore and remeasure the choke. Clean, re-oil and readjust your reamer and repeat the process until the desired choke size has been reached.

Cutting a choke with a hand adjustable reamer requires a bit of practice, but if you keep your reamers sharp, they will cut with minimal effort and leave a very smooth finish. You should never exert very much inward pressure on the reamer; let the reamer find its way down the bore, and very little effort will be required to turn the tap handle. Remember that the object is to get as smooth a cut as possible to minimize the amount of polishing needed to bring the choke to a decent finish. With the reamers that I use for this operation, I do not adjust more than one full turn per cut, and as I approach the desired finished size, I adjust the reamer only 1/2 turn.

American Gunsmith, Book of the Shotgun. Chapter 16

Courtesy, American Gunsmith

Here are a number of reamers that are used for making fixed-choke adjustments. The two on the left are adjustable hand reamers; the three on the right are Clymer’s carbide adjustable choke reamers. They are intended to be power driven.

Choke Adjusting On The Lathe

Adjusting a choke’s exit diameter on the lathe is a very simple operation using the Clymer carbide choke adjusting tools. Because of the carbide cutting surfaces of the Clymer tool, this method also has the advantage of cutting through chrome finishes that are often found on the bore surfaces of imported shotguns, without the tedious task of first grinding away the chrome.

To begin the job, measure both the bore and choke diameters of the shotgun barrel and write them down. Subtract the choke from the bore to determine the constriction. The Clymer choke-adjusting tool uses the same bronze bore pilots as both Clymer and Colonial (type 2) choke tube installation tooling. Determine which bronze pilot best fits the bore. This is most easily done by trial and error. Insert the pilot through the chamber and up the bore, until it stops against the choke forcing cone (noting carefully how far its front edge is from the muzzle). There are three possible positions on the tool’s drive end on which to locate the bronze pilot. The pilot’s position, relative to the choke-forcing cone, will determine its correct location on the tool shaft.

The next step is to mount the barrel on the lathe, with its bore centered to the lathe spindle. This is easily done by firmly inserting a chamber mandrel into the barrel chamber and installing this assembly into the three-jaw self-centering lathe chuck. Raise the lower jaw of the padded milling vise (attached to the tool post) to meet the bottom of the barrel. Tighten down the upper vise jaw. Loosen, then retighten, the bolt controlling the tilt of the vise. Now loosen the three-jaw chuck slightly and move the apron to the right. Remove the chamber mandrel and insert the tool extension handle through the lathe spindle. Place the chamber guide bushing over the drive handle and attach the choke adjusting reamer to the handle with the roll pin that is provided with it, and slide this assembly up the bore to a point about a quarter of an inch before the choke forcing cone. Since there will be a certain amount of “crush” to the barrel where it is mounted in the milling vise, it will be necessary to slightly loosen the vise in order for the tool and pilot to pass between its jaws. Retighten the vise and tighten the three-jaw chuck. Expand the choke-adjusting reamer slightly. This is done with a 5/32-inch allen wrench inserted through the muzzle. Oil the muzzle and tool and run the lathe at approximately 75 rpm. Slowly feed the barrel over the rotating tool with the apron handwheel until the reamer exits the muzzle. Move the barrel back to the right; stop the lathe and remeasure the choke exit diameter. Adjust the tool for the next cut by turning the allen nut and proceed until you have reached the desired constriction. I like to make the final cut using the lathe’s power feed set at approximately .012 inch per revolution. This results in a very smooth cut that needs little polishing.

American Gunsmith, Book of the Shotgun. Chapter 16

Courtesy, American Gunsmith

A dial bore gauge is the fastest way to determine the size of a shotgun bore.

Polishing the parallel surface of the fixed choke can be done in any number of ways. I do not advise bringing this surface to a high polish. A good crosshatch pattern left by a medium hone stone seems to work best. If you check the muzzles of some of the best side by side shotguns made, you will generally find this cross hatch pattern or even reamer marks left during the final sizing. Often the gunmaker did his final choke sizing at the range and stopped all sizing and polishing operations as soon as the desired pattern was reached. It is thought that this slightly rough surface improves choke performance.

If your reamers are well oiled and sharp, and care is taken in the cutting of a choke, your polishing job will be quite simple. A ball-type brake cylinder hone attached to a hand drill should do the job nicely in just a few minutes. These hones are available from most tool stores or from your local auto parts store. I like to flood the area that is being polished with honing oil and move the hone in and out rapidly as it is spun at a relatively high speed by the electric hand drill. A two-stone brake cylinder hone can also be used. In my shop, I like to use the Sunnen bushing grinder to bring the choke to its final size and finish. This takes very little time, but care must be taken not to remove too much material. With light stone pressure and a flood of honing oil, move the barrel in and out slowly. A little practice will soon give you the feel of a well honed surface. I like to turn the barrel over a few times during final sizing on the Sunnen hone. Perhaps this is just superstition, but I was once told by an old gunsmith that this will ensure that the honing operation is done evenly.

American Gunsmith, Book of the Shotgun. Chapter 16

Courtesy, American Gunsmith

A ball-type brake cylinder hone on a hand drill can be used to finish a choke.

Also, a hardwood dowel spun by the lathe can be used. Make a 3-inch slit in one end of the dowel with a hacksaw and wrap emery cloth clockwise (as you face the slit end) around the dowel. A little experimenting, and you will find the correct length of emery cloth to do the job. Insert the dowel from the chamber of the barrel while it is spinning at a relatively high speed. Do not insert the dowel from the muzzle end of the barrel. This might cause a slight bell shape to form at the end of the choke. The “tool” should not fit too tightly in the choke and should not be permitted to completely exit the muzzle during the polishing operation. Again, care must be taken with this method not to remove too much material.

If you equip your shop to handle choke adjusting, and follow these instructions, you will soon find choke adjusting to be one of the most popular services you perform.