An apparatus for pressure burst testing of a hollow vessel, such as a gun barrel, includes a hydraulic cylinder assembly and a test mandrel mounted on a frame in axial alignment. The test mandrel includes a hollow sleeve stationarily mounted on the frame and having an outer end adapted to receive thereover the vessel such that the portion of the vessel to be tested extends beyond the outer end. An elongated drawbar is disposed coaxially within the sleeve and is attached to a reciprocally driven piston of the cylinder assembly. At an outer end, the drawbar extends beyond the sleeve outer end and is coextensible within the vessel portion to be tested. The piston and drawbar therewith are movable relative to the sleeve through respective pressure-generating and pressure-releasing strokes for causing radial expansion and contraction of a resiliently deformable annular member, preferably formed of one or more ring-shaped elements of elastomer material, and thereby respective application and release of pressure by the deformable material against the interior of the vessel portion to be tested. The deformable member is mounted on the drawbar and disposable within the vessel portion to be tested. Also, the deformable member is located between the sleeve outer end and the drawbar outer end such that movement of the drawbar relative to the sleeve causes the respective pressure-generating and pressure-releasing radial expansion and contraction of the deformable member.