Gun Reports - Special Reports
American Gunsmith: Secrets of Screw-In Choke Installation
February 14, 2012
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No doubt the biggest change in the shotgun market today is the acceptance of the screw-in choke. It took a long time for the American shotgun owner to warm up to this device. Winchester has been offering them for more than 30 years, starting with the Model 59 semiautomatic in 1959. Mossberg introduced its Accu-Choke in 1978 but, until recently, most shotgun owners never really saw a need for them. After all, double-barreled shotguns already had the immediate choice of two chokes. It wasnt uncommon for a Remington 870 owner to also own a sizable collection of additional barrels, which werent all that expensive.
In recent years, just about every shotgun manufacturer has made screw-in chokes available for some of their products. Some have stopped making fixed choke barrels altogether. The American shotgunner has been thoroughly sold on the benefits of this feature. Many gun owners wouldnt consider a shotgun that didnt have screw-in chokes. The fixed choke gun may be going the way of the manual transmission, black and white television and the typewriter.
The increased acceptance of new shotgun sports has also had its effect. Card shooters (remember when they were called turkey shooters?) now take their favorite game much more seriously. It isnt uncommon to find one of these individuals with a pocket full of choke appliances in 0.005 inch increments, trying one after another in an attempt to master that days distance, weather conditions, and ammunition. Sporting clays participants wouldnt be caught dead with a fixed-choke shotgun. After all, every gun specifically made for their game has screw-in chokes.
Other target shooters have also adopted the device. Trap shooters have found they can use the bottom barrel of their over/under shotguns, no matter at what distance they are placed at handicap events, and thus limit recoil and sight plane distortion caused by heating the upper barrel. They can also make adjustments in choke constriction to compensate for ammunition changes. The skeet shooter has also benefitted. It isnt uncommon to see the better skeet shooters practicing with relatively tight chokes, and with the increased popularity of doubles events, many skeet shooters have found an advantage to using a tighter choke constriction on their second shot.
What about the poor waterfowl hunters? Many have had their favorite shotguns legislated out of the game now that we all have been forced to use steel shot. Whats to become of all those old L.C. Smiths, A.H. Foxes and Model 12s that fit like gloves and brought home the legal limit time after time, year after year? Surely some of these will be traded in for the latest 3-inch magnum, screw-in choked, steel-shot safe, offerings available, but not mine! My favorite old Model 12 is now threaded to accept thin wall chokes. When duck season opens, Im out there with the shotgun I grew up with. The only difference is that there is an extended steel-shot choke tube installed; so if damage is going to result by my using this abusive new ammunition, Ill throw away the choke tube and screw-in another. My fowling piece is safe.
I know Im not unique in my attachment to this old Winchester. There were millions of shotguns made before the fixed choke fell into disfavor, and most of the gun owners who now possess them have every intention of keeping them.
The bird hunter who hunts everything in season has found he can become a better shot and a better hunter if he always uses the same shotgun and the same barrel length. Public acceptance of screw-in chokes is so great that I have many wholesale customers who convert used shotguns in their inventory to include this feature.
So if card shooters, sporting clays shooters, skeep and trap shooters, hunters and shotgun owners in general all agree on the benefits of the screw-in choke, and millions of guns have been made with fixed chokes, how big do you suppose the need for screw-in choke installation is?
It is enormous! You should install screw-in chokes.
What Youll Need
First of all, youll need tooling, and tooling is expensive. Youre going to have to decide between the Win-Choke, the Rem-Choke, the True-Tube and Thin-Wall systems (I realize there are more options out there, but these are the most popular and you will find choke appliances readily available for these systems).
The Win-Choke and Rem-Choke systems require an outside barrel diameter of 0.845 inch in 12 gauge, and similarly large diameters in the smaller gauges. A quick check of several fixed choke barrels you might have around the shop will show you that these systems will fit in relatively few barrels. The True-Tube system requires an outside diameter of 0.825 inch. With this system, youll be able to choke many more barrels. With the Thin-Wall system, youll be able to choke barrels down to 0.805 inch. You should find very few barrels smaller than this.
The temptation here will be to buy only the Thin-Wall tooling in 12 gauge, thinking youll be able to machine every possible barrel with this system. However, another parameter that must be considered is maximum bore diameter. The True-Tube system works in bores up to 0.736 inch internal diameter. The Thin-Wall system will be safe in bores up to only 0.730 inch. Again, a check of barrels around your shop will show that many of these are over the safe maximum diameter for Thin-Wall installation. My suggestion here is that since 12 gauge is the most popular shotgun gauge today, make a large investment. Buy both the True-Tube and Thin-Wall tooling. In the long run, youll be able to make your tooling pay for itself faster if youre able to do every job that comes through the door.
Now consider the other gauges. Personally, I do them all. But at a minimum, you will want to be able to do 20-gauge installations as well. Again, by checking the 20-gauge barrels around your shop, you will find that the Win-Choke and Rem-Choke systems require a relatively large external diameter. The True-Tube system will allow you to install chokes in many more 20-gauge guns. This is the one you want.
Youre going to need choke-adjusting reamers. The angle blade reamers found in the Brownells catalog are an excellent choice, and you probably have these already. Youll also need internal measuring equipment. An inside divider and an outside micrometer will work very well, but there are other methods available. Youll need polishing equipment. A hand drill and brake cylinder hone will suit your needs. Youll need a drive adapter. Well cover these later.
I realize Ive just suggested an investment of almost $1,200. Unfortunately, Im not finished yet. In order to do the best job, youll need a relatively large inventory of choke tubes. You may want to keep as many as 100 on hand to start. Your initial investment in the screw-in choke business may be as much as $2,000. If youre still with me, Im going to show you how to make this one of the best $2,000 investments you ever made.
Installing By Hand
There are many manufacturers of screw-in choke installation tooling, but I will limit this instruction to those brands offered in the Brownells catalog. Tooling available from your favorite toolmaker will be easily adapted to these instructions. The tooling available from Brownells is made by either Clymer Manufacturing or Colonial Arms. They both offer very high quality and accuracy. There is, however, one major difference in their construction. The Colonial Arms reamer has an added feature in that it removes the existing choke as the initial step in the machining process. With the Clymer tool, you have to remove the choke with expanding reamers before machining. This, of course, will not be necessary if shortening the barrel is going to be part of the job. The first step in the choke installation process is to measure the barrel outside diameter. Lets say, for example, that we have an L.C. Smith shotgun barrel that has been sawed off, thus eliminating the problem of having to remove an existing choke. With our outside micrometer, we determine the barrel to be 0.810 inch. This will require a Thin-Wall choke system. Check to see that the bores are centered in the barrels. Youll be surprised how many are not, but well talk more about this later. The L.C. Smith, a shotgun of very high quality, has its bores perfectly centered in barrels.
Now we have to measure the bore diameter. Clean the bore thoroughly to ensure accurate measurement. Checking the manufacturers specifications, we find a maximum bore recommendation of 0.730 inch in the Thin-Wall system. If we exceed this parameter, the result will be that the choke itself will be smaller than the bore at the seating area, the wad and shot column will snag at this point, gas pressure will get behind the choke appliance and choke failure is inevitable. This is not pretty! The barrel will most likely bulge at the machined area. I have seen this happen. The last thing you want to have to do is buy a new barrel, so measure carefully. The easiest way to measure is to try to insert the maximum pilot. In the case of our L.C. Smith, the 0.730 inch pilot will not go into the bore. It can safely be choked. Trying additional pilots, we find that a 0.728 inch pilot fits snugly down the bore. This is the one well use for both reaming and cutting threads. We attach the 0.728-inch pilot to our reamer.
Now we secure the barrel in a padded bench vise, making sure that weve left enough barrel exposed so
We are now ready to attach our source of hand power, the good old fashion bit-brace. Colonial Arms makes bit-brace adapters for both their own and Clymer tools. These adapters have a floating drive to insure that the reamer follows the snug-fitting pilot acurately down the bore. Insert the piloted tool and bit-brace into the first bore to be choked. Squirt plenty of high quality cutting oil (I use Brownells Do-Drill) over the tool and down the bore. (Did I mention that it is a good idea to place a drip pan on the floor?) Lean into the bit-brace end knob with your stomach. This should be at about belt level. While keeping a careful eye on the floating drive end of your bit-brace adapter to be sure it stays in the center of its possible travel, and while applying steady and relatively heavy inward pressure on the whole assembly with your stomach, turn the bit-brace clockwise. Youll be surprised at how quickly the reamer cuts when used in this manner. Go no more than 1/4 inch at a time. When youve reached 1/4 inch, withdraw the tool while continuing to rotate it clockwise. Never rotate any reamer counterclockwise. Brush off the chips, reoil both the tool and the bore and continue the operation. Keep reaming until the shouldered end of your tool is 0.040 inch to 0.050 inch from the end of the barrel. This is not a critical measurement and Colonial Arms supplies a barrel seating spacer, which is nothing more than a flat washer about .045 inch thick, to put over the tool for those of you who need help in recognizing 0.040 to 0.050 inch. After youve done six or eight choke installations, youll find no need to use this spacer. You are stopping the tool short of completing the job in order that you can cut the final choke seat after youve cut the threads. This will insure that you have a perfect choke seat with no marks on it that may be caused by touching it with the tap. This whole reaming operation usually takes 10 to 12 minutes.
I like to keep an old 5-gallon paint pail, with about 3 gallons of cleaning solvent in it, under the bench. Remove the tool from the bore and from the bit-brace adapter. Holding the drive end, shake it around in the solvent a few times and remove the pilot. Remove the barrel from the vise and shake the muzzle around in the solvent. This will clean it sufficiently for the tapping operation.
Squirt a good amount of cutting oil on the reamed shoulder that will be threaded. Attach the same 0.728 inch pilot that you used to guide the reamer to your tap, and oil the tap. I have never attempted to tap choke threads under power of any kind, and I dont recommend it. Using a Colonial Arms floating tap wrench, start cutting the threads as evenly as possible, again keeping an eye on the tool holder to be sure the floating section stays in the center of its possible travel at all times. Cut the threads completely, without withdrawing the tool until youve hit the choke seat lightly. When the threads are complete, I like to back out the tap halfway, wait a few minutes for the barrel to cool and run it home a second time. Back out the tap all the way, remove it from the tap wrench and shake it around in the solvent. Remove the pilot and put it back on the reamer.
Now its time to cut the last 0.040 to 0.050 inch. This last cut will take you just about a minute. Clean the newly machined and threaded area of the barrel (spray carburetor cleaner works well,if you have good ventilation in your shop), oil the threads and install the choke. Run a mop down the bore from the chamber and inspect your job.
Looking down the muzzle under a good light, you want to see a little of the choke seat, all the way around. This indicates that the barrel diameter is slightly less than the diameter of the choke opening and the wad and shot will pass this point with no snag.
There, youve done your first screw-in choke installation in about half an hour.
Using Power Hand Tools
It doesnt take a genius to figure out that the bit-brace is a tiresome apparatus to use on a regular basis. It can easily be replaced by a half-horse hand drill. Heres how to do it:
Put your bit-brace adapter in the vise and drive out the roll pin that connects its two halves. I realize you probably dont have a lathe, or you wouldnt be using a bit-brace or hand drill, so take this operation to a friend or a machine shop if you have to. Use a lathe to turn down a 6-inch piece of round stock to 0.435 inch. Leave about an inch of the round stock at this diameter and turn an additional 3-inch section down to 0.375 inch. Now cut off your 4-inch part and grind or turn the ends smooth. One quarter inch from the larger diameter end, drill a 13/64 inch hole perpendicular to and straight through the center of your piece. Install this part in place of the bit-brace center piece with the roll pin, and you are ready to attach your adapter to a hand drill.
Follow the instructions for installing by hand, but be careful to keep the tool speed under 75 rpm and the tool itself very well oiled. Again, be sure to keep the floating center of your tool adapter in the center of its possible travel at all times.
You will find using a hand drill is much faster than using the bit-brace and a lot easier on your arms.
Installing Chokes On Lathes
There are two basic methods of installing screw-in chokes using the lathe. The reaming can be done between centers or by using a tool-post-mounted milling vise. The advantage to using the milling vise is that youll also use your lathes power feed, and this certainly makes the reaming operation much easier. So lets start with the milling vise.
A very good milling vise is made by Palmgren and available through the Brownells catalog. Any lathe owner will find many uses for this tool. I use mine for back-boring, porting, forcing cone lengthening, and installing screw-in chokes. Mount the milling vise over the tool post on your compound rest, using an inverted tool holder to clamp it down. You want the V-cut in the lower vise jaw parallel to your spindle.
For initial alignment of your milling attachment, chuck a piece of straight round stock about 15 inches long and about 3/4 inch in diameter in the three jaw chuck. To be sure your setup is accurate, center your alignment bar with your tailstock center. Now adjust the cross slide and the lower jaw of the milling vise until the V-cut in the lower vise jaw contacts your alignment bar. Tighten down the upper vise jaw. Loosen, then tighten the bolt controlling the tilt of the vise. Remove your tailstock and set it aside. Now loosen the three jaw chuck slightly. You should be able to move the lathe apron laterally, moving the alignment bar in and out of the three jaw chuck without the bar hitting the jaws. You have just roughly centered your milling vise.
Lets say, for example, we are installing chokes in a Remington 870 barrel. It is 28 inches long with a modified choke and a vent rib, and the owner wants to keep it the same length. If you have Clymer tooling, you will have to start by reaming out the existing choke with expanding reamers. For our example, however, we will use the Colonial Arms tooling.
Measuring the outside diameter of the barrel, we find it measures 0.835 inch. We will be installing the standard True-Tube choke system. Now we measure the inside diameter. Clean the bore thoroughly to insure accuracy. Measuring can be done by inserting pilots from the chamber end until we find the size that fits snugly within the bore. An 0.730-inch pilot fits ours. Checking the maximum bore specification tells us this barrel can be safely choked since the maximum bore diameter is 0.736 inch for standard True-Tube style chokes. Slide the pilot up the bore from the chamber to the choke forcing cone. You will need a long shaft to do this (an old golf club shaft or wooden dowel will work). Thread the reamer to the pilot tightly and take this assembly to the lathe.
With the jaws of the milling vise open wide, put the drive end of the tool in the three jaw self-centering chuck and tighten the chuck for now. With protective padding in place (I use half of an old canvas shot bag), adjust the cross slide and the lower vise jaw to meet the bottom of the barrel, with the V-cut in the lower jaw centered along the barrel. Be sure to leave enough of the barrel exposed to the tool so that the pilot is not allowed to enter the area of the barrel within the vise jaws. Close the milling vise loosely and rotate the barrel counterclockwise (as you face the spindle) until the vent rib just touches the upper vise jaw. This will insure that the barrel does not rotate
Now we attach the floating adapter. The same adapter that we made to hold the reamer in the hand drill is placed between the three jaw chuck and the tool. In this way, our drive is free-floating and the machining operation will follow the bore by way of the properly fitted pilot. We are now ready to begin reaming.
The manufacturer recommends that these reamers be rotated no faster than 75 rpm. Unlock the spindle and engage the backgear to run at low speed. I like to run my reamers at 60 rpm. Set the feed rate at 0.0066 inch per revolution. Squirt a lot of high quality cutting oil over the reamer. Start up the lathe and engage the power feed friction clutch. Your reamer is now removing the fixed choke in the 870 barrel. Once youve run the choke removing section of the reamer halfway in, loosen the automatic feed friction clutch and move the apron to the right. Be careful not to pull your pilot into the choke. Brush off the chips and reoil the tool. Now the first step on the reamer is entering the bore. From this point on, cut no more than 1/4 inch before stopping to clean and oil the tool. Continue machining until your reamer is 0.030 to 0.040 inch from finishing. You will cut this remaining amount after the threading operation.
Remove the tool from the bore and from the drive adapter. Be careful not to disturb the adjustments on your cross slide or milling vise. You will be putting the barrel back on the lathe after the threads are cut. Shake the tool around in the pail of solvent and remove the pilot. Shake the barrel around by touching it with the tap. Put the pilot on the tap and secure your barrel in a bench vise, again making sure that you left enough barrel exposed to the tool so that your pilot will not enter the section of barrel held within the vise jaws. Threading is done by hand, following the same instructions given in the earlier section.
When threading is complete, return the barrel to the lathe and set it up for final machining. This will take no time at all since the milling vice and cross slide have not been disturbed. Machine the last 0.030 to 0.040 inch, until the stop shoulder on the reamer just contacts the muzzle. You have just cut the choke seat, insuring that there are no marks on it caused by touching it with the tap.
Screw-in a choke and inspect your job under a good light. Again you want to see the face of the choke seat all the way around to be sure that the bore is slightly smaller than the entrance of the choke.
With very little practice, you will be able to install a choke in about 15 minutes using this method. Using the power feed on your lathe has made this job incredibly easy.
If you have a long lathe, you may want to machine barrels between centers. You will need some sort of chamber mandrel to attach the chamber end of the barrel to your tailstock. There are expanding mandrels available, and Brownells sells a chamber mandrel with a lock pin to prevent the barrel from turning around it. Attach your mandrel to your tailstock and place the barrel over it. If you are using a mandrel that requires a lock pin, insert the pin. Put a dead center in your headstock for setup and a steady rest at the halfway point of your barrel. Adjust the steady rest with the muzzle aligned in the headstock center then slide the tailstock to the right and remove the center.
Some gunsmiths mount their reamer to the headstock in a collet but I prefer to use some sort of floating adapter. Colonial Arms provides these in Nos. 1, 2 and 3 morse taper.
Set the lathe to run at or below 75 rpm and manually feed the barrel over the reamer, using the tailstock handwheel. This is very much a matter of feel. Dont try to cut too quickly. Apply steady and even pressure with the handwheel and dont forget to stop every 1/4 inch to clean the chips and lubricate your tool.
This method is just as fast as using the lathes power feed and, with a little practice, your setup time will be only a few minutes.
Youre going to find a lot of shotgun barrels with chrome bores, especially in guns made in Europe or Japan. If the barrel is choked Skeet, or if shortening the barrel is part of the job, removing the chrome is very easy. Just sand the face of the muzzle, either by hand or with a power sander using the miter table, to remove the chrome from this surface. Your reamer can now be used safely, since the first step will be contacting the bore below the level of the chrome.
If you have a fixed choke and must maintain the barrel length, youll have to remove the chrome from the choked section of the barrel as well. This can be a very tedious job if you rely on a brake cylinder hone for removing the chrome. Heres an quicker way:
Chuck a piece of 5/8-inch hardwood dowel that has been slit with a hack saw about 3 inches down its center in your headstock (your drill press will also work). Insert one end of a piece of coarse emery cloth 3 inches wide by about 8 inches long in the slit and wrap the rest clockwise around the dowel. Experiment with the length of the emery cloth to get a tight fit in the choke. Do this by tearing off about 3/8 inch at a time until it fits. Lube the choke with penetrating oil and, with the sanding fixture started in the bore, turn on the power while holding onto the barrel. The area being sanded will get very hot during this operation, so dont hold it near the muzzle. Remember, you only want to remove the chrome from the constricted area, so dont go in any further. Dont forget to remove the chrome from the face of the muzzle.
If you are installing a lot of screw-in chokes, and do a lot of fixed choke adjusting, you should consider investing in a Sunnen sizing hone. These machines make short work of chrome removal and they can usually be obtained quite cheaply. Ask your local automotive machinist where to find one; they are the same machines used to size connecting rods and king pins.
This method of removing chrome is the fastest Ive found and should take only a few minutes. Most shops charge a little extra when chrome has to be removed before choke installation.
A lot of shotguns (Rizzini, early Berettas, Franchi, American Arms, etc.) have been imported with screw-in choke systems for which additional chokes are difficult to obtain. Often, the Rem-Choke system can be installed in these barrels. Measure the original choke tube O.D. and note the distance from the last (muzzle end) thread to the choke seat. If you find that the Rem-Choke is larger than the original system, you can run your Rem-Choke installation tube right over the old installation. On some barrels, you will have to shorten the tubes about 1/8 inch in order to start the Rem-Choke threads beyond the originals.
The Winchester Super-X shotgun has been very popular. On the later models, and on most high rib models, the vent rib is retained at the front rib post (often by an unsightly Allen screw) and floats freely everywhere else. I have found that this retainer is not strong enough after a screw-in choke has been installed in a Super-X barrel. The front rib post will be ripped out of the barrel by the weight of the recoiling vent rib, nd the vent rib will slide rearward across the top of the receiver. To assure that this does not hapen, either retain the vent rib in the top of the receiver by means of a long shanked steel 6x48 center bead. You may also drill the side of the rib at two of the rib posts near the center of the barrel, using a #52 drill, and secure the rib to the posts with 1/16-inch roll pins. The latter method is my preference because it takes very little time and I have not seen one of these repairs fail.
Youre going to find a lot of bores that are not even close to being in the center of the pipe. Always look for this when the owner is present. After youve choked the barrel, your choke installation will also appear to be off center. This is no time for the owner to discover this condition. Hes going to think your installation is the problem and that the gun will no longer shoot straight. Explain to him from the start that your machining is fully piloted and accurately follows the bore of his shotgun barrel. The shotguns point of impact will not change.
There are also safety concerns when a bore is off-center. Measure off center bores very carefully to be sure that you will not leave one side of the barrel dangerously thin.