Hundhammer's studies were interrupted in the summer of 1918 by World War I, in which he served briefly on the Western Front before enlisting in the Freikorps and taking part in the battle for Munich against the Bavarian Soviet Republic in April 1919.

Hundhammer received his doctorate in 1923 with a thesis entitled "Die Geschichte des Bayerischen Bauernbundes" ("The History of the Bavarian Farmers' League).

One of the results of his campaigning on behalf of the farming class was the published work Staatsbürgerliche Vorträge (Civic Lectures) in which he strongly rejected the tenets of National Socialism from a thoroughly Catholic viewpoint, labeling it the "new Germanic Hedonism.

His assets and attempts to find qualified work were both blocked by the Nazis, so he acquired a shoe-repair workshop in the fall of 1933 with the help of Father Rupert Mayer in order to continue providing for his growing family.

""Mindful of the physical devastation which the survivors of the Second World War were led into by a godless state and social order lacking in all conscience a respect for human dignity, firmly intending moreover to secure permanently for future German generations the blessing of peace, humanity, and law, and looking back over a thousand years and more of history, the Bavarian people hereby bestows upon itself the following democratic constitution.

He founded the Katholischer Männerverein Tuntenhausen in 1945, remained active in the Catholic student groups he had joined during his time at the university, and became a member of the Order of the Holy Sepulchre in 1957.