The Avengers was made at Elstree Studios - the studio, which was in dire straits financially, like most of the British film industry at that time, only seems to have had a small number of guns as the same ones turn up again and again. A Beretta M1934 (they seem to have had two of these as there are occasionally two in the same shot) and a Colt Dick Special are the most common. A P38 often shows up, and occasionally a Polish Radom/Viz when they needed something extra. Patrick MacNee hated guns, and repeatedly said so, despite having worked in North America for most of his career and becoming a US citizen in the 1960s. His character frequently avoided using them.