Precision Barrel Fitting, Part Five

In this issue we talk about barrel crowning and extractor cuts. A rifle crown, while not regal and jewel encrusted, serves a vital purpose. It must do its part in assuring every point on the circumference of a bullet base exits the muzzle at exactly the same time. A properly cut crown will accomplish this mission. An improperly cut crown will not, allowing propellant gas to escape from one side of the base while its opposite side is still engaged with the bore.

The result: Bullet tipping upon exiting the bore and potential keyholing at the point of impact. Do you know howa keyhole hit is scored on a bullseye target? At the lowest scoring ring it contacts upon impact with the paper. Suppose you fired such a shot in a drum head tight match. One end of your bullet struck the nine ring, the other cut into the ten ring. What do you get? A nine. I don’t think anyone in the world knows for certain how many competitions have been lost by a single point.

Well, perhaps there are records kept of such happenings at National, Olympic and other International shoots going back to the Creedmoor days. Never having acquired the ability to control my heartbeat prior to pulling a trigger, I’ve avoided extending a handgun or shouldering a rifle in any one of them. Keyholing could bring about personal bodily harm if the object struck is a large game animal.

I’ve heard of a reasonably placid moose being instantly transformed into a charging, enraged bull moose in mating season when a 180 grain .30- 06 stung him sideways instead of An improperly crowned rifle is a rifle subject to accuracy problems. With large game the problems can result in disaster for the hunter. The lathe crowning tool shown within the circle is used for 11 degree crowns. The most frequently used factory crown (A) is more difficult to cut than the recessed target (C) and 11 degree (D) crowns.

The final angle of any crown cut must be 90 degrees to the bore axis. entering head-on and below his left shoulder. Make no mistake, crowns on rifles are important. There are four to choose from. Which is the best crown to cut? What is the lathe set-up for muzzle crowning? The second question is the easiest to answer and is illustrated here. As for the first question, it depends. The conventional factory crown is called a radiused crown.

Though its appearance might suggest otherwise to be true, the factory crown is not as easy to cut as a recessed target crown or 11 degree crown. The latter is favored by many hunters. The recessed target crown and 11 degree crown styles work well for both hunting and tar-

Illustration 61 shows how to use masking tape to help prevent barrel run out in these set-ups. A more secure means is assured in Illustration 62 by drilling and tapping four holding screws in the spindle. The best way to prevent run out is with a barrel centering assembly at both its ends (Illustration 64). get rifles. Some military target rifle competitors prefer the 11 degree on the theory it excels all others in uniformly controlling propellant gas at the muzzle, improving accuracy.

This may be true but the enhancement is usually only discernible in top level competition and some long range varmint rifles. What do I consider "long range"? Anything beyond 100 yards. What do I consider a varmint? Raccoons prowling about in daylight and likely to be rabid, chipmunks who think a free standing stone wall exists for their burrowing desires, crows in a friendly farmer’s field intent on devouring his money crop, mad dogs, wild house cats… and jihadists.

Any rifle, well zeroed, can bring all their actions to a quick end at 100 yards or so. Out beyond is where the preferred 11 degree crown could produce the extra accuracy required. Factors like wind, humidity, and temperature become more critical with distance. Both can and do have a negative effect on accuracy. I’ll bring up a little first hand experience to help make my point. The rifle was a nicely scoped bolt action Sako in .223, MOA at 100 yards and having often proven its worth many times at that range.

The scope had been rezeroed on a six-inch diameter paper plate hung on a bush at 200 yards. Around was touched off at a prairie dog sticking its head outside its burrow to one side of the bush. The bullet hit about six inches short and the dog ducked back into its hole untouched. Soon afterward, a tin camp cup replaced the escaped head and a second shot let go from 200. The cup flew four feet into the air. Forget wind, humidity, temperature, and all other factors.

I know the real reason I missed that first shot: lacked control of my heart rate. To date, sadly, no progress to reverse that inadequacy has been made. Back to crowning. The lathe crowning tool shown within the circle in illustrating crowning the muzzle is used for 11 degree, target or recessed target crowns. Tool angle is not critical but something else is highly critical. The final angle of the crown must be 90 degrees to the bore axis.

Finishing cuts will make the crown very pleasing to the eye and these cuts should not exceed 0.002- 0.003 each. Illustrations 61, 62 and 64 show alternate methods of setting up a barrel in a lathe for crowning. In each example the barrel is introduced through the lathe spindle. The spindles found in many small to medium size lathes are of insufficient internal diameter to make the intrusion possible. Choose wisely if the initial machine tool purchase is limited to less than full size.

In any barrel through head stock operation, particularly in crowning, the work piece must be centered in both spindle and lathe chuck. If they I aren’t, or do not remain centered, the crown will not be cut at the required 90 degrees to the bore axis. One of the ways centering goes awry is bar-

rel run out, or slipping horizontally, within the spindle. The easy way to help prevent barrel run out and a poorly cut crown is shown in Illustration 61. Wrap the barrel with duct tape to create a press fit between barrel exterior and spindle interior. More better is fabricating and using a centering bar. It has one end tapered to support the chamber and works extremely well in maintaining the work piece during through-the-spindle setups.

Details of howa centering bar assembly is applied and modified to secure it appear in Illustration 64 and greater details on fabricating a centering bar assembly are shown in Illustration 65. Its shaft must be turned to fit inside the head stock spindle. Tightly. Other specifics appear in the drawing. Most if not all the components can be worked up from materials available at a local hardware store. For sure, the store will carry rounds of cold rolled steel for the bar shaft.

There is some doubt the same will be true of brass stock for the turning and internal threading of the centering point. And I have no doubt whatsoever your local hardware store carries absolutely no inventory of solid plastic balls. Don’t fret. A golf ball makes the ideal substitute and a used golf ball is satisfactory. You can uncover dozens of them hiding in the rough at the nearest country club, which is good as brand new golf balls have gotten too blankety-blank expensive anyway.

I sense you have spotted the ready-made centering bar assembly from Brownells identified in the drawings I’ve mentioned. Anything wrong with it? Not that I’ve heard and if there were the assembly wouldn’t have made it into John’s book. I further sense you might still be a bit shy of DIY machine tools. Entering into this conversation once again are the gangs from Montezuma and Dave Manson. He’s developed and produces a high quality tool for hand crowning a muzzle. You heard that right. No lathe.

There are cutters with 90, 45, and 11 degree angles available with interchangeable pilots. You don’t have to totally tear down a rifle to use them but you cannot skimp on cutting oil and should avoid bearing down with all your might in the actual cutting.

Clean the cutter and pilot frequently and keep the bore clear of fine metal In this configuration it fits inside and is secured to the spindle at the rear, seats within the breech at the front and combines with a muzzle-end centering bar from Brownells to maintain the workpiece aligned true during through the spindle operations.

Developed by Dave Manson, the tool is available with 90, 45, or 11 degree cutting ends, interchangeable pilots and gives you no excuse for bearing down heavily to produce crowns fit for a king. particles that will scratch it. Using the Manson Muzzle Crown tool will deliver crowned barrels that’ll make you proud. When not using it, store it and it accessories in a padded container. Right now it’s time to return to machine tools. Milling Barrel Extractor Cuts Some barrels require one cut. Others will require two.

Still others will have one or two slots. As you are about to find out, cuts call for a setup considerably different than the setup for slots. Both these critical excavations will vary in size from narrow in width and from deep to shallow in depth as dictated by the extractor dimensions. Do I hear, "Hold up there, Chick ol’ fella, what if I don’t have a milling machine?" That doesn’t necessarily chase you off the job.

If you have a wide enough assortment of files and the dexterity, cuts and slots can be made by hand. I’ll suggest another, and perhaps more preferable, route you can take.

The setups for end mill and rotary saw cutters both use the lathe milling attachment. As you can see, the rotary cutters also require a DIY milling arbor. As you can also see, the arbor will not be difficult to fabricate. Ever heard of a Palmgren Lathe Milling Attachment? It’s an excellent substitute for a dedicated milling machine, a lot of pounds lighter and more than a whole lot less expensive. Industrial supply houses, such as MSG and Grizzly Industries, offer them.

The attachments come in three sizes or models. They all fasten to the compound rest and hold the barrel in a small vise. The vise and barrel can be moved up and down via a screw adjustment graduated in thousandths of an inch. Longitudinal movement is controlled via the lathe carriage, transverse movement via the lathe crossfeed and, when required, the attachment can be swiveled 360 degrees. The latter allows setting the vise jaws at a right angle to the lathe bed or any angle in between.

This lathe milling attachment is a high quality machine tool delivering limited, but very flexible, milling capabilities to lathes. Depending on the application, it will require an end mill or rotary saw-type milling cutter. End mills are available in single or double end conformation and a wide range of diameters, lengths, and shank sizes. Shanks are usually by fractional size in 1/16th-inch increments. The best way to secure an end mill in a lathe is with a collet and collet closure.

Second best is a four-jaw chuck indicated to hold the mill within 0.0005 on centers. A three-jaw chuck unable to repeatedly match 0.0005 is not recommended. Setup vs. Actual Machining In a majority of machine operations, setup time exceeds actual machining time, sometimes considerably. In Illustration 67, for example, the barrel is clamped in the milling attachment vise parallel to the lathe bed so its breech and the end mill face one another. The cross feed will move the barrel back and forth across the mill.

Cut depth can be determined by using a dial indicator positioned against the carriage. It is seldom if ever possible to purchase an end mill that precisely matches the width of the extractor. This greatly reduces the possibility of being able to make a matching cut in a single pass.

Your solution to that problem is to use an end mill smaller than the cut width dead centered on the final cut width, make one pass, recenter the end mill, then widen your initial cut with initial passes on both sides of the center line until the extractor width is equaled. Arbor For Milling Cutters Arbor holes in rotary saw cutters are standardized. Those with one-inch holes have the greatest variety of thicknesses and diameters to choose from.

Following those facts, the arbor must be turned to through a one-inch hole. The turned arbor must be long enough for threading. The threading alone has to accommodate two large nuts, two large diameter flange washers, and the cutter. A milling arbor turned to fit smaller arbor holes could be used but adherence to the larger hole enhances rigidity and minimizes vibration. Next time we dive deeply into chambering and other aspects required to make a rifle accurate… and safe. AG