By 1905, the army constructed five powerful batteries at Fort Barry that represented the new Endicott-period upgrades: Battery Mendell, Battery Alexander, Battery Smith-Guthrie, Battery Samuel Rathbone and Battery Patrick O’Rorke. Battery Mendell would be the first gun constructed in 1901, the purpose of Battery Mendell and the four other fortifications were to fire 1,100-pound artillery shells at enemy ships up to eight miles away. When the guns were ready to fire they would rise from their positions fire a single shot at the enemy target then recoil down into a hidden position where the guns and soldiers manning it would be protected by a large concrete parapet.
Between 1922 and 1938, there was no permanent garrison assigned to Fort Barry. But in 1939, on the eve of World War II, the post was officially reactivated and troops were again assigned to man the batteries. Fort Barry now fell under the jurisdiction of the Harbor Defenses of San Francisco. Because of advances in military aviation, the military could now expect enemy fire not only from ships but now also from the sky, in the form of accurate bombers and strafing fighter planes. As a response, anti-aircraft guns were installed along the coast to protect the uncovered and vulnerable batteries from aerial attack. Along with the threat of chemical attack, all bunkers were made gas proof. By the fall of 1942, fears of an immediate enemy attack faded. The alert warning for the coastal defenses was downgraded to a level “B,” which allowed men to return to their barracks as long as they could reach their battery stations in 15 minutes. Between 1943 and 1946, Fort Barry’s battery armaments were removed and scrapped and in the fall of 1946, the last of the Coast Artillery troops left Fort Barry.