Installing Screw-In Chokes Without a Lathe

[Updated January 28, 2019]

Maybe it’s the economy, or simply the urge to upgrade a favorite scattergun, but I’ve been getting many more requests recently to install choke-tube systems in shotguns that were purchased before choke tubes existed.

In cases where the gun under consideration belonged to Grandpa, was passed on to Dad, I rarely undertake the modification. Today’s shotguns use 1040 steel in their receivers. Many older shotguns used a milder steel, such as 1020, which isn’t as strong and may present a danger under the pressures developed by modern loads. This isn’t to say an old gun will go to pieces the instant a high-brass shell is ignited in its chamber. It may not happen for another 1,000 rounds. It may never happen. But if it does, the injured party’s lawyer will file a liability suit at the nearest county seat. I don’t want my name on it. And, I personally can’t stand the thought of ruining the value of a fine old Parker or Purdey by installing screw-in chokes and refuse the job on principle.

Until a few years ago, choke work required you to have a lathe with a bed long enough to accommodate a shotgun barrel. If you weren’t so equipped, the only way to accomplish the job was to send the gun to someone who was, perhaps another gunsmith or aftermarket suppliers like Hastings, or Colonial Arms. The latter two offer choke-installation services with trade discounts.

Though the lathe remains the favored method among those doing a great deal of choke work, it is no longer the only way. Simple hand tools, devised by Colonial Arms and Clymer Manufacturing of Rochester Hills, Michigan, have made choke-system installation possible without a lathe.

This tooling is not cheap, and you must purchase it by gauge and type of choke tube system being installed. I suggest you do a bit of research and purchase only the tooling that best suits you after discussing your needs with the sources listed here.

Since I have some of Colonial’s 12-gauge tooling, here’s how to use it. The basic procedures are similar with any tooling you choose to buy.

Taking the Measurements

After making sure the gun is unloaded, remove the barrel, thoroughly clean the bore, and keep in mind during your measuring process that some manufacturers expand the end of their barrels. The order in which you take your measurements doesn’t matter, but take them all.

For choke-tube installations, the bore diameter of a shotgun is measured internally from the end of the barrel’s forcing cone to the point where the choke begins at the barrel’s end. If no choke exists, the bore diameter is measured from the end of the forcing cone to the end of the barrel. The standard bore diameters for U.S.-manufactured shotguns are as follows:

12 Gauge, 0.729 inch

16 Gauge, 0.667 inch

20 Gauge, 0.617 inch

28 Gauge, 0.550 inch

.410, 0.410 inch

All of the above are +0.020 inch, meaning, for example, that the bore diameter of a 20 gauge could measure out to 0.637 inches and still be considered standard. These bore-diameter differences that exist between manufacturers make no difference, however, when determining the amount of choke that exists in a fixed choked shotgun.

Regardless of gauge, when a shotgun’s business end has been constricted, or choked, the amount of that constriction is deducted from the bore diameter to provide the amount of choke. Constriction of the various chokes are as follows:

Full, 0.040 inch

Improved Modified, 0.030 inch

Modified, 0.020 inch

Improved Cylinder, 0.010 inch

Cylinder Skeet #1, #2, 0.003 to 0.010 inch

The actual choke diameter of a shotgun is the bore diameter, which can vary, plus the amount of constriction, which doesn’t vary. This should help explain why a full fixed-choke 12-gauge barrel with a bore diameter of 0.745 inches may not pattern as tightly as the same choke in a barrel with an 0.735-inch bore diameter.

Before You Start

There are a couple of simple ways to find out whether the internal bore diameter of the shotgun will allow choke installation. The first is to select the steel pilot included with the tooling kit that represents the maximum size for the gauge. Insert the pilot from the breech end of the barrel. If it drops easily into the bore, the bore is too big to be fitted with choke tubes.

Simpler still is to use a Colonial maximum bore gauge. These tools, available according to gauge, are inserted into the chamber and past the forcing cone. If the gauge continues into the bore, the barrel cannot be fitted with choke tubes. As a rule, you cannot install tubes in barrels with an internal diameter (ID) that exceeds 0.781 inches for 10 gauge; 0.736 inches for 12 gauge; 0.730 inches for 12-gauge thinwall; 0.668 inches for 16-gauge; 0.626 inches for 20 gauge; 0.560 inches for 28 gauge; and 0.416 inches for the .410 shotgun. Forging ahead regardless will definitely cause choke damage and the distinct possibility of barrel blowout.

Before installing choke tubes you must also determine whether the wall thickness of the barrel after reaming will be sufficient to assure safe operation. Do this by measuring the outside diameter (OD) of the barrel at the place where the choke tube(s) will be installed with a reliable set of dial calipers. Next, measure the OD of the tap you’re using. Subtract the OD of the tap from the OD of the barrel and divide that number by two to establish the barrel’s remaining wall thickness. If the resulting figure indicates the wall thickness will be less than 0.010 inch, do not attempt installation under any circumstance.

Another element that must be included in your calculations is the concentricity of the barrel’s outside diameter with its inside diameter. If not concentric, the barrel will be thinner in some spots than in others and the problem of insufficient wall thickness could rear its ugly head. It will definitely come up if anemic areas are present immediately behind an existing, fixed choke or where the installation is to be performed on an unchoked gun. You may or may not be able to make allowances. For example, the minimum OD for a 12 gauge, 0.795 inch-by-44 (thread size) Colonial choke tube installation is 0.825 inches. Say the OD of the barrel on your bench measures 0.834 inches and its ID is thicker by 0.004 inches on one side than the other. Dividing that variance by two equals -0.002 inches and the adjusted OD becomes 0.0832 inches, which is larger than the minimum required OD of 0.0825 inches.

How do you measure OD and ID accurately and simultaneously? A Clymer-made barrel-wall thickness gauge allows direct measurement in 10- through 20-gauge shotguns and is equipped with a dial indicator. The barrel to be checked out is slipped over the gauge’s contact ball and the indicator’s plunger lowered to contact the barrel.

Both hands are used to move the barrel up and down between the plunger and the ball to give a direct reading of thickness at the point of contact. Barrels up to 32 inches long can be gauged by measuring one half, turning the barrel around and measuring the other half. When using this tool, you never put too much pressure on the arm of the fork with the contact ball on its end. This can cause the arm to flex slightly and give a thinner than actual read- out. As accurate as it is, you still have to be cautious in using this gauge, or whatever means you may use, when measuring Belgium Browning over/unders. There may not be enough metal between the barrels for choke tube installation even though your calculations tell you they’re perfect.

Reaming and Tapping

Slide the barrel seating spacer over the reamer until it stops on the reamer’s largest dimension. The spacer is a small, but very important, part of choke work because it will stop the reamer before the final choke seat is cut. As you’ll soon discover, forgetting to put it on can really louse things up.

Next, check to see if the barrel to be fitted with chokes is chrome lined. If it is, you must remove all chrome from its muzzle end before going any further. If we were employing a lathe, the chrome could be removed by facing the barrel. Since we’re not, wrap a piece of 220-240 grit emery cloth around a flat piece of steel or a broad, flat file and sand off the chrome. Check your progress with cold blue, which colors the bare steel but not chrome. When you’re finished, thoroughly clean off the emery grit to avoid damaging the reamer.

If the barrel has an existing, fixed choke and hasn’t been cut off to eliminate the choked area—which can be accomplished with a hack saw or chop saw—use one of the five steel pilots that come with the tooling. They’re graduated in 0.002-inch increments. The one you select should fit snugly into the bore with no wobble yet still turn freely. Choosing a pilot that’s too small will allow the reamer to wander out of line with the bore during cutting operations and cause changes in the gun’s point of impact. Double check your choice and bear in mind that the next biggest pilot to the one you select, if there is one, shouldn’t fit into the bore at all. The chosen pilot is inserted from the chamber end of the barrel, its threaded end toward the barrel, and pushed gently down the bore. It will come to a stop when it reaches the existing choke; don’t try forcing it further. Now screw the end of the reamer into the pilot through the choke.

If the barrel has no existing choke or has been cut off, use a bronze bushing that fits the bore without wobbling but still turns freely. Like the steel pilots, five are included with the kit. They’re also graduated in increments of 0.002 inch so all the guidelines mentioned for selecting a pilot apply equally to bushing selection. However, you don’t drop or push the bushing down the bore in this situation. Instead, place it on the end of the reamer, insert it into the muzzle and make sure the reamer’s snap ring is in place. It acts as a stop for the bushing.

You never use a bushing in combination with a pilot and you never use a pilot in combination with a bushing. Those in the kit will do most jobs but others are available and can be ordered from either Colonial or Brownells.

When it comes to the actual cutting, you have some options. You can use a special adapter to link the reamer to an adjustable speed hand drill run at no more than 50-60 revolutions per minute. You can use a different adapter to connect the reamer—or tap—to a hand brace. You can also use a hand held tap wrench. Like the adapters, it is hinged at one end and allows the reamer or tap to find its own center and prevent misalignment. It’s called a floating tap wrench and is recommended here because, in my opinion, it will give you a better “feel” for reaming and tapping barrels the first few times you do it. Now it’s time to remove some material.

Lock the barrel in a padded vise to prevent slippage, then brush or squirt a little cutting oil on the reamer and into the bore. Securing the barrel toward its muzzle will help it stay put and it won’t flex as much as it would if you locked it up at its chamber end. If you’re using a steel pilot, you’re ready to start reaming. If you’re using a bronze bushing, you obviously have to insert the tooling assembly into the muzzle. Use plenty of pressure and torque at all times.

Turn the reamer clockwise about 1/8 inch into the bore. Remove it, still turning it clockwise. Clean all the chips off both the reamer and the barrel, relubricate both and make another 1/8 inch cut. Never, and I stress never, turn the reamer counterclockwise during any phase of the cutting operation. There’s almost no better way to permanently damage an expensive piece of equipment. Continue cutting, removing, cleaning and lubricating until the reamer stops because the barrel is rubbing on the seating spacer. After removing the reamer assembly, give the barrel another thorough cleaning to clear it of chips before you tap it.

Internal threading on the barrel must be done by hand after you remove the spacer and install the tap on the same pilot or bushing used for the reaming. If it’s a bushing, the snap ring must be in place. Lube the tap with a good quality cutting oil and start it slowly into the bore. Though it will cut fairly easily until it bottoms out on the choke seating surface—about 17 full turns—never force the tap beyond its stopping point. It is capable of removing the seating surface under excessive pressure and no seating surface means a poor installation or no installation. When threading is complete, be careful in removing the tap from the barrel to avoid damage to the threads and clean the bore again.

Cutting the choke seat is next. Leaving the barrel spacer off, switch back to the reamer and insert the tooling assembly into the bore. Clean and lube the reamer and turn it clockwise until it faces off the end of the barrel. Remove the reamer clockwise, clean out any chips and use a bore light or flashlight to check your work. The threads you cut in previously should end above the seat you’ve just finished cutting and a small gap should be visible between them. Giving in to the temptation of threading this unthreaded portion in a misguided attempt to make the choke tubes “fit deeper and tighter” will damage the seat.

Finishing Up

Remove the barrel from the vise and give it a thorough cleaning with a bronze bore brush and quality solvent. Put a little choke lube on an improved cylinder tube and install it in the barrel. It should screw in smoothly and fit flush with or a little below the muzzle when it has seated. Now put a light up the barrel, checking to be certain the seating edge of the tube is resting squarely against the bore, which is smaller in size. It will look like a ring. If it doesn’t, a dangerous situation has been created . You can, and should, make a simple tool to double check this visual inspection. Take a 6- to 7-inch piece of welding rod or a piece of coat hanger and sharpen one end to a point. Put a 90-degree bend about 1/4 inch long in the sharp end and use the tool as a feeler to confirm the presence of the tube seat. The feeler should catch in the seat on the way in and pass over the tube on the way out.

Remember, improperly seated choke tubes can catch shot, wad, or both, and be torn out of the gun. Be sure that “ring” is at the interface between tube and bore with every choke tube you install. I mean every tube, not just the IC, and test fire the gun before returning it to your customer. If you have doubts—and you may have some the first few times—mount the shotgun in a rifle vise, clamp the vise to the bench and tie a long enough string on the trigger to keep you well back from the firing line. If you’ve messed up, it will cost you a new barrel. If you mess up and leave the testing to your customer, it will cost you a whole lot more.

A few, related, afterthoughts about sights seem appropriate about now. If the barrel you’ve choked is ribbed, there’s little to be concerned about during reaming and threading as long as you don’t crunch the rib in the vise. If you’re working on a plain barrel with just a single a bead, you don’t have to remove it because it will probably fall out on its own during the cutting operations. There might have enough thread left on the bead for you to reattach it with epoxy laced with powdered steel. Sometimes it stays on, sometimes it doesn’t. You can also replace a bead sight by soldering on a ramp drilled and tapped to accept a new bead. If you plan to do so, however, you must do your soldering before reaming and tapping. The heat from a torch can warp a previously machined barrel and its threads in the process. Whether the barrel will require reblueing depends on how good and careful you are when it comes to sweating on ramps. The most profitable alternative may be to replace any lost sight by mounting a Polychoke rib.

Before tackling your first choke installation job, though, I seriously recommend a few practice runs. Scrounge around for an old barrel, cut about 10 inches off and use both ends of the piece to develop your skills. Finally, always use choke tube lube. It contains metal particles to support the threads. And periodically check the unloaded gun to make sure the choke tube is fully seated.