That Key West experience
“Military Men standing by a small gun. Fort Zachary Taylor. Key West. 1918. Monroe County Library”

Looks like a 3-inch gun on a masking parapet mount without the gun shield (which would have gone on the hooks) mounted. Taylor had six of these guns in two batteries (Adair and Dilworth) between 1899-1920 during the installation’s Endicott Period, which would correlate to the uniforms, which curiously are Naval though the fort was an Army Coastal Artillery post. Perhaps they were just checking out the landlubber’s gun…
From the position, it looks like Battery Adair, which mounted four Driggs-Seabury low-angle 3-inchers in M1898MI mounts, emplaced to cover controlled minefields leading up to the fort’s masonry walls. The battery was named after the late 1st Lt. Lewis D. Adair, 22nd U.S. Infantry, who died 5 Oct 1872, of wounds received in action with Sioux Indians at Heart River Crossing, Dakota Territory. Adair, who at the time of his last battle was fifteen miles from Heart Butte, on Heart river, while on duty with his company escorting the Northern Pacific Railroad survey of the area, was reportedly given his death blow by the great Hunkpapa Sioux chief Gall.
According to Fort Wiki, the end of the Great War ended the battery’s usefulness and “On 27 Mar 1920 all four guns were ordered removed and the carriages salvaged. The guns were transferred to Watervliet 17 Sep 1920 and the mounts were scrapped 20 May 1920.”