The SMI opened its own factory in Campo Tizzoro, a hamlet in the municipality of San Marcello Piteglio in the Tuscan-Emilian Apennines, in 1911 to manufacture cartridges. In the following years, the first housing complexes were built for the workers, turning Campo Tizzoro into a factory town. 

With the outbreak of the Second World War, the factory increased its production of ammunition, and an extensive network of tunnels and air-raid shelters was completed underground. During this period, the first clandestine anti-Fascist organisation began to form, which became fully active after the German occupation. Inside SMI, a partisan formation was established: the Campo Tizzoro Patriotic Action Squad. In addition to workers, the group included some clerks and several local women. 

Their first activities were to help and rescue Allied prisoners who had escaped from internment camps, stray soldiers and those who refused to enlist in the Republican army. Throughout the year of German occupation, there were continuous acts of sabotage against machinery and the removal of cartridges. Subsequently, especially during the spring and summer of 1944, numerous armed actions were carried out against fascists and Nazis. 

In 1944, approximately 6,800 people worked at SMI, who were managed by engineer Kurt Kayser. A memorial stone near the factory is now dedicated to Kayser, for not hindering partisan activities, for working to avoid reprisals and for preventing the destruction of the company during the German army's retreat to the Gothic Line. 

The factory was liberated and occupied by the partisans on 26 September 1944, shortly before the arrival of the Allied troops.

SMI in Campo Tizzoro continued its activities after the war and, until the 1980s, was the heart of the economy of the Pistoia mountains, combining the production of bullets with the processing of bronze, steel and copper laminates. In 2012, the SMI Campo Tizzoro Museum and Shelters (Museo e Rifugi S.M.I. Campo Tizzoro) was opened, however as of 2023 it is closed to the public.