The term gun drilling is applied to the process of drilling a hole along the center axis of a solid rod or bar that has a known outside diameter. Conversely, the hole may be drilled and the outside is machined for consistent wall thickness around the circumference. Occasionally, drifting of the drill will cause wall thickness eccentricity. It is important in the gun drilling process to monitor the position of the drilled hole to ensure that drifting has not taken place.
With ultrasonics it is possible to measure eccentricity of wall thickness by scanning around the circumference of the barrel or by manually placing an ultrasonic transducer at various locations around the barrel. Either method will provide the wall thickness information required to ensure that the gun drilled hole is within concentricity specifications. Measurements with ultrasonic gauges are nondestructive, accurate and dependable.

Equipment: Manual measurements to monitor eccentricity in gun drilling can be done with small ultrasonic thickness gauges. Wall thickness and diameter of the barrel ultimately determine the selection of the instrumentation. In general, the Model 35DL and Model 38DL PLUS gages are well suited for wall thickness measurements.
As a rule, size of the contact transducer should increase with the size of the barrel. In some cases when the diameter of the drilled hole is small compared to overall diameter (and the wall is thick), it is necessary to use a focused immersion transducer. Because of the many variables involved, it is recommended that actual samples be tested in order to select the correct instrument and transducer combination. The most challenging situations involve large diameter parts with very small drilled holes.
Scanning of the barrel may improve speed of measurements and increase the total surface that can be monitored. Scanning for eccentricity should be done through water immersion. Small diameter barrels can be scanned for eccentricity by using the RBS-1 Recirculating Bubbler System. Large diameter barrels need special fixturing.