CSP Bore Micrometer

When I have the opportunity to write a review on a shotgun, one of the areas of information I like to include is the choke constriction as it relates to the bore size of the shotgun in question. After all the years I have been involved with smoothbores, it still amazes me how many shooters do not understand or actually comprehend that choke value is only determined when the bore size is known. So many gun owners think all they need to know is the size of the muzzle exit. Somehow they do not get it that you must have the internal diameter of the bore as well as the I.D. of the muzzle (choke area) in order to determine the difference between the two, the actual number of points of constriction of the choke.

And although there may be a number of ways of measuring these areas that will get you close, there is really only one good way to do get this information — a high-quality bore micrometer. Often called a bore reader, a bore micrometer is a precision tool designed to measure the internal diameter of a machined hole in metal, wood or other reasonably hard materials. They are ideal for the purpose we speak of here. The brand of bore micrometer I use is made by CSP of Sturgis, SD, and marketed by 100 Straight. I asked Bruce Bowen about the evolution of his bore mic. Here is his story.

“When I first began working on and modifying competition shotguns some 45 years ago, I was often frustrated by the lack of a good quality, versatile measuring device for internal barrel geometry. This is one of the least understood, but most important, aspects of shotgun shooting. I felt the only way to get what I needed and wanted was to simply build the device myself. I began with different designs but eventually ended up producing a bore micrometer that did everything I felt it needed to do. It was accurate, easy to use and could measure at least six different gauges: 10, 12, 16, 20, 28 and .410 bore. Originally, all parts were made on manual lathes, mills and grinders. During the past 20 years, components have been made using CNC machining. The use of CNC machines has enabled us to have better consistency, better accuracy and appearance. The gauge heads are interchangeable on our bore micrometer, this requires the thread fit to be as accurate as possible, and the CNC machining has been perfect for this. The use of precise thread gauges when machining shafts and the heads ensures a head manufactured today will fit properly on a unit manufactured long ago.

“To prevent against rust and corrosion, as well as improved appearance and durability, the shafts and heads are hard-chrome plated. Gauge rings are black oxide finished. Overall, the instrument hasn’t changed much since the original design except in appearance and perhaps better accuracy. The most recent improvement has been a high-quality oak box providing improved protection and storage as well as protection for the unit during shipping.

“A micrometer would not be worth much if its precision and accuracy weren’t up to snuff. Our micrometers are calibrated during the manufacturing process with Brown & Sharp plug gauges and rings. Our rings are consistent to within +.0005 -.0000. The gauge heads are precise to within +/-.0005 over the range of the head. The larger gauges measure over a range of approximately 100 thousandth of an inch, and the smaller gauges somewhat less. For example, the 12 gauge measures from approximately .650" to .750".

“The heads of our unit are marked with the gauge size, and the rings are marked with the corresponding diameters for easy recognition and use. Example: To measure a 12-gauge barrel, you would install the 12-gauge head on the shaft and slide the .700 ring over the three steel balls on the head. Next, set the dial on the indicator to 0. The 0 represents .700. Inserting the head into the muzzle end of the barrel past the choke area will give a reading larger than 0. For example, 29. This indicates a bore diameter of .729 which is the nominal size for American shotguns and many European shotguns as well. As you pull the head toward the muzzle, the dial will read a lower number and might even go past 0. For example, it might read 95. This would indicate a choke diameter of .695 and thus a Full choke. To determine the actual points of constriction the choke is imparting on the pattern, you would subtract .695 from .729 for a total of .034.

“The shaft of the instrument has graduation stamping up to 6" inches on it. This allows you to read how far the balls on the head are into the barrel.

“Over the last 25 years or more, it has become increasingly popular in the USA to overbore competition shotguns. This process is commonly referred to as ‘back boring’. It is usually an aftermarket process performed on European shotguns, because many countries have proof laws that prevent bore sizes larger than the nominal size. Older American shotguns are often over-bored as well. This complicates measuring the choke with the traditional dime method, because you are measuring the diameter of the end of the barrel and not the bore size. Often a dime will go right into a 12-gauge barrel that has a very tight Full choke. Remember, choke constriction is the difference between the bore diameter and the choke diameter. Barrel-wall thickness also becomes a safety consideration in this process. I have seen many barrels dangerously thin from over-boring.

“Included with the bore micrometer is a concise instruction manual, which includes charts for choke, chamber and bore sizes. Special sizes are available. These bore micrometers are used by gun manufacturers and gunsmiths worldwide. They are available with both digital and analog indicators. For metric measuring, the digital model is recommended.”

So, whether you are a professional gunsmith, gun builder, or talented home-trained gunsmith, at some point you will need an accurate and durable bore micrometer. The bore micrometer from CSP is an excellent choice. You can reach them at 605-347-2232 or on the web at www.100straight.com. Be sure to tell them you read about them in Shotgun Sports. SS

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