A high-definition gamma scanning method for the near-field measurement of radionuclide inventories in a large nuclear waste barrel is presented. The method introduced is especially accurate for radionuclides with multiple gamma energy peaks. Multiple detectors positioned as closely as possible to the waste barrel are used to measure the radiation field emanating from the distributed radiation sources. The total source activity is reconstructed by using the conjugate gradient with nonnegative constraint method or the maximum likelihood expectation maximum method based on measured detector responses. The maximum measurement error bond and its associated confidence level for the developed gamma scanning system are determined statistically by performing a large number of numerical experiments that take into consideration the counting statistics, the nonuniformity of source distribution, and the heterogeneous density of the self-absorbing medium. The accuracy and reliability of the system are verified through a series of real measurements with randomly distributed 192Ir sources in a 208-litre waste barrel. The results of these measurements are in full agreement with the estimated error and the confidence level that are predicted by the numerical simulation.