Finish-crowning the rifle barrel: the "major" recessed or concave barrel crown is only the beginning of the job. For good accuracy, the rifle or pistol barrel also requires "finish" crowning as a necessary part of the overall muzzle finish.
Common sense tells us that there must not be any abrasions or rough edges surrounding the periphery at the muzzle where the bullet makes its departure. What I call "finish crowning will take care of these rough edges, and can be easily applied to new firearms (when required, as is often the case) or to any firearm where the muzzle undergoes major crowning during routine barrel work.
With even the sharpest lathe bit, the recessed cut at the major crown will leave rough edges at the barrel mouth, making finish crowning a necessary step. Fortunately, finish crowning is easy to do, requiring only a few basic items found in most shops, and very little time or skill is required to do a perfect job.
Performing the Finish Crown
To perform the finish crown, you'll need a small electric hand drill, some kind of a crowning ball, and a mild abrasive paste. I've found that J-B Bore Cleaner (sold by Brownells) works fine as an abrasive. For the crowning ball, all you need is a roundhead, slotted brass wood screw or machine screw of proper size, cut down with the shank remaining. Screws in sizes 8, 10, 12, and 1/4 inch will handle most of the bores you'll get through your shop.
As a ballpark size, I use a screw-head diameter of 0.300 inch for 22 caliber to 6mm, 0.350 inch for .25 caliber to 7ram, and 0.395 inch for .30 caliber up to 8mm. For larger or smaller calibers than those listed, just be sure the crowning ball is at least 0.030 inch larger than the bore being crowned.
To get started, place the crowning ball (round-head brass screw) in the electric drill chuck and apply a liberal amount of the abrasive material both to the screw head and at the mouth of the bore. Hold the barrel upright and, with gentle pressure, apply the rotating screw head squarely against the bore mouth. Align and hold the drill as straight as possible to the barrel, and slowly rotate the barrel as the drill is turning. After applying the crowning ball for 15 or 20 seconds or so, remove the drill and wipe off the case mouth. Carefully inspect the muzzle crown with high magnification to see how the bevel is forming. Then apply the rotating crowning ball as the barrel is turned until a nice smooth bevel is completed. For a good finish-crowning job, I like to see at least a 0.020-inch bevel at the edge of the muzzle.
Then as a final measure, I lap-polish the bore mouth using a tight patch coated with J-B Bore Cleaning Compound on a cleaning-rod jag. The entire bore should then be cleaned in preparation for test firing. As test shots are fired, you'll notice the petal-like powder fouling patterns that form at the periphery of the crown. These should be nice and even as they form.
At a cost of less than a dollar and a few minutes of your time, a professional finish crowning can be applied to any gun barrel. As a final measure to assure accuracy and prevent muzzle damage, all barrels should have this treatment.
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|Title Annotation:||Back to Basics|
|Author:||Johnson, Norman E.|
|Date:||Feb 1, 2008|
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