1. n. [Perforating]

The distance between the external surface of the gun assembly and the internal surface of the casing or liner. The gun clearance depends on the position of the gun within the tubular and will vary between phases of shots on any gun unless the gun assembly is centralized. This variation in clearance contributes to variable performance of the perforations. The greater the gun clearance, the smaller the entrance hole-with the effect increasing with depth (pressure).