The star gauge is an artillery tool used to measure the true diameter of a bore at numerous points. It is used as part of the inspection procedure both at the arsenal and in the field to check the bore for acceptance during its initial fabrication and for wear and imperfections after the gun is in service. Similar gauges are still in use today.
This very rare and finely machined piece of tooling is stored in a custom fitted, dovetailed, walnut toolbox measuring 59 inches X 7 inches X 4˝ inches. The lid is affixed by three brass hinges and two pivoting hook fasteners. There is a 3 inch X 2 5/8 inch brass plate inset into the center of the lid which reads: U.S. / FRANKFORD ARSENAL / 1859 / STAR GAUGE. The brass gauge handle, muzzle caliper, and a brass ruler bear the same marks. Glued wooden blocks and interlocking loose blocks are used to hold all of the internal parts. A walnut tray that holds the measuring points, multi-tool, and screw fasteners, is lined in dark burgundy velvet. The gauge consists of one section with the head attached and numbered 13 to 42 (inches) plus two extension tubes marked 43 to 82 and 83 to 122. The tool-steel muzzle caliper assembles into a T and is used to center the handle end during insertion into the cannon tube. The steel measuring points are screwed onto the distal head of the brass shaft (three each at 120°). The steel centering assembly is marked: 6 Pdr., 12 Pd.r, 18 Pdr., 24 Pdr., 32 Pdr., 42 Pdr., 8 Inch, 10 Inch, and 13 Inch to accommodate those gun sizes. There are forty-two measuring points, some being extras. There is a wooden block formed pocket for an unknown (and missing) 7 ˝ inch X 2 ˝ inch X 7/8 inch device.
On page 68 of THE ARTILLERIST'S MANUAL by Brig-Gen. JOHN GIBBON, he describes the way a star gauge is used. The example he describes differs in some details from the example in-hand. He states: STAR GAUGE, Fig 40 and 41, PL. 11. - To ascertain this [the size of the bore], a more complicated and delicate instrument is used, called the star gauge from the shape of its head, which is of brass, with four steel sockets, two movable and two stationary, for the measuring points. There are four measuring points for each calibre; and when two of these are screwed into the fixed socket, the distance between their points is equal to the true diameter of the bore. The movable sockets rest against the inclined sides of a slider or wedge whose sides incline 0.35 in. in a length of 2.2 inches, so that by pushing the slider the 35th part of this distance (about0.06 in.) the distance between the two sockets or the measuring points, if screwed into their place, is increased .01 in. The slider is fastened to a square steel rod consisting of three parts, which are screwed together according to the length of the bore to be measured. This rod passes through a brass tube, which is also made in three parts, and to screw together. This tube is graduated into inches and quarter inches, commencing at the plane of the measuring points, so as to indicate the distance of these from the muzzle of the gun. The handle, Fig.41, Pl.10 is of wood, attached to a brass cylinder or socket through which the rod passes into the handle. The socket of the handle slips over the end of the brass tube made smaller for the purpose, and has a slit in it allowing the brass tube to be seen through. On The side of this slit a scale constructed, to indicate the movement of the measuring points. Each joint of the long tube has a mark on it, to show the position for the zero of the scale when the instrument is properly adjusted for any particular calibre. In this position the handle is fixed to the sliding rod by means of a screw clamp. A REST, Figure 42, PL11, in the form of a T, is placed in the mouth of the gun to keep the instrument in the axis of the piece. It has three sliders, which can be adjusted on the different limbs, to suite any size bore. Commencing at the muzzle, the diameter of the bore is measured at intervals of a calibre, as far as the trunnions. From that point to the seat of the shot, a diameter is measured at every inch, and for every quarter of an inch, for the rest of the bore. No variations over 0.03 of an inch are allowed.