Measure a Barrel From Groove to Groove
Procedure for barrels with an even number of grooves.
|Use a brass case to make a gauge for measuring the groove to groove diameter of a barrel. Brass will not damage the crown or inside surface of a barrel when making and using the gauge.|
|Flatten a section of the front of the case in a bench vice.|
|A new or annealed case works better than an old brittle one and, of course, the case does not have to be one that is used for the caliber measured. We need the gauge to be brass or aluminium. A case is chosen because it is at hand.|
|Barrels with an even number of grooves are easier to measure than barrels with odd numbers of grooves. The barrel used here is a six groove barrel. The excercise is to measure from groove to groove as shown.|
|File a flat section from one side of the flattened case. Do not file from both sides; the side that remains folded, holds the brass together.|
|Check the width of the gauge against the groove diameter of the barrel and, when the gauge is close to the right width to go inside the barrel, do the work below.|
|File two angles on the leading edge of the gauge. It allows the gauge to start into the barrel and gives a better feel of when the size is getting close to the right width. Use a very fine file or emery paper to polish the gauge until it fits the barrel with a bit of resistance.|
|The final polishing, to get the gauge to slide into the bore with just a bit of drag, must be done very carefully and slowly. Rather take just one or two strokes at a time at the gauge, between fittings, so that it comes to a precise fit.|
|Once the gauge slides into the bore with no play and a little bit of drag, it is exactly the same width as the barrel diameter from groove to groove.|
If you do not have a digital micrometer capable of reading at least three decimals in millimetres, go to an engineering company that has such an instrument. A digital vernier does not have the resolution for this type of measuring. Measure the gauge for the groove to groove diameter of the barrel.
To get the land to land diameter of the barrel, the same procedure is followed, it is just a bit slower and a bit more frustrating because the lands are not as wide as the groves and it is easier to slip off a land.
Once a gauge is made, it is worthwhile to check around the barrel from groove to groove, to see if the opposing grooves are all the same diameter. The same applies to the opposing lands.
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