Guys,
Here is a tool and technique to check and make sure that the bore is on center in a barrel. If the bore is off center, it can be easily be corrected by using a German milling machine (a big coarse tooth file). I use this mostly when putting the swamp into a forged barrel. Of course, it is easy to get the bore on center at either end where you can easily see, but what about in the middle?
The tool is made from two 3/8 inch diameter rods, 24 inches long, joined by a block so the rods are 1 inch apart, center to center. One rod end uses a replaceable jag/patch to center in the bore with an adjustable sliding conical fitting to center at the bore end. The other rod has an adjustable screw to help to determine the distance from the bore center to the barrel outside.
Here is an overall photo of the tool assembly.
Here is a photo of the joining block (#10 screws)
Here is a photo of the bore centering conical fitting (#6 set screw).
Here is a photo of the replaceable jag and the adjustable screw (with a set screw to hold it, both #6)
Here is a photo of the tool installed in a 42 inch, 0.513 caliber barrel. I am checking the waist area of the swamp. A moderately tight patch is on the jag.
Here is a photo of the conical centering fitting at the muzzle end. It helps to use the tool vertically so this fitting is held to the bore center by gravity.
Here is a photo of the adjustable screw almost touching the waist area of the swamp. Each flat is checked by rotating the tool (in the direction to tighten the screw threads, of course). By eyeball you can quickly check the bore center within a few thousanths of an inch. If it is not on center, get the big file and correct it. Always be sure there is a tiny visible gap between the adjustable screw tip and the barrel outside. With a 24 inch reach the tool can check to the middle of a four foot barrel, assuming access from both ends.
Jim
