The method is based on SPME (Solid Phase Micro-Extraction) sampling from the atmosphere inside the shotgun barrel. Two analytical systems were used in parallel, the GC/TEA and GC/FID(MS) system. These two systems detect various combustion products, and the estimation of time since the latest discharge is based on the rate of escape of the volatile discharge residues from the barrel as a function of time. If the ammunition used in the suspect firearm is known, the use of decay curves for the actual firearm and the actual ammunition is fairly straightforward. For unknown ammunition, repetitive samplings must be performed and the results fitted to decay curves obtained for other ammunition types according to the proposed procedure. Generally, the method presented in this study can give an indication whether a weapon was fired, for example, 2 to 3 days, 1 to 2 weeks, or more than 3 weeks ago. Some factors that may impact data interpretation were studied, such as the effect of cleaning the barrel, the number of shots fired, the eventual disturbance to the system by SPME sampling, and the effect of storage temperature. 13 figures and 10 references