6-inch gun M1897

After the American entry into World War I, the Army recognized the need for large-caliber guns for use on the Western Front. The Coast Artillery operated all US Army heavy artillery in that war, due to their experience and training with these weapons. A total of 95 6-inch coast defense guns were removed from fixed emplacements or drawn from spares and mounted on M1917 wheeled carriages as field guns; most of these (72, plus possibly a few ex-Navy weapons) equipped three Coast Artillery regiments in France, the 61st, 62nd, and 68th.[7] The guns were nicknamed "6-inch Terrors". However, due to the Armistice, none of these regiments completed training in time to see action. By 1917, pedestal mounts for 6-inch guns (all of them M1900 weapons) were known to be superior to disappearing mounts, being able to more rapidly track targets with a faster rate of fire. Thus, most disappearing guns (except the M1897, shorter than the others) were dismounted for use as field guns, while most of the few pedestal guns dismounted were returned to the forts soon after the war. The removed 6-inch disappearing guns (primarily M1903 and M1905) were stored and many were returned to service in World War II. The Army weapons removed included up to 18 M1900 guns and 74 M1903 and M1905 guns based on carriages ordered (M1917A carriages for the M1900 weapons, M1917 carriages for the M1903/M1905 weapons). One source states that four M1900 guns and 68 M1903/M1905 guns arrived in France.[5] An additional 46 6-inch guns of other types were provided by the Navy and 30 ex-Navy guns from arms dealer Francis Bannerman; a few of these were possibly delivered to France before the Armistice. These included Navy guns Marks 2 through 6, of 30, 40, 45, and 50 calibers length. All of the Bannerman guns were 30 calibers long; the number of guns of other lengths is unclear. Sources state that all Navy guns were cut down to 30 calibers barrel length in an attempt to standardize ballistics, as that was the length of the shortest Navy guns.[5][8][9][10][11] Thirty-seven M1917B carriages were ordered for the Navy guns, with a view to having a spare tube for each carriage; it is unclear how many were produced, or if any were delivered to France.[5] Some of the Army weapons (primarily the M1900 guns due to their fast-operating pedestal mounts) were returned to coast defenses after the war, but most (a count of disarmed batteries shows approximately 81)[12] were stored until World War II. One survives on a field carriage in the collection of the U.S. Army Ordnance Training and Heritage Center, Fort Lee, Virginia. In June 1919, after the Treaty of Versailles was signed, the field carriages for the 6-inch guns were declared obsolete and almost entirely scrapped.[5]

6-inch gun M1905 on shielded barbette carriage at Fort Columbia State Park, Washington state

Rear view of shielded barbette carriage

Battery 245 at Fort Stevens, Oregon, two 6-inch guns on shielded barbette carriages, built in World War II. The battery's ammunition and fire control bunker is behind the gun.

Typical entrance to 6-inch ammunition bunker at Fort Ebey, Washington state

Along with other coast artillery weapons, some of the 6-inch guns in the Philippines saw action in the Japanese invasion in World War II. Since they were positioned against a naval attack, they were poorly sited to engage the Japanese, and the open mountings were vulnerable to air and high-angle artillery attack.

In 1940–44, 16-inch gun batteries were constructed at most harbor defenses to replace the aging Endicott- and Taft-era weapons. Many 6-inch weapons (most of them stored since World War I) were remounted on M1 through M4 shielded barbette carriages at new locations in two-gun batteries to complement the 16-inch guns. These allowed higher-angle fire than previous mountings, and extended the 6-inch guns' range from 17,000 yards (16,000 m) to 27,000 yards (25,000 m). M1903 and M1905 weapons were remounted as the M1903A2 and M1905A2, and a new M1 gun (initially designated the T2) armed some batteries. A heavily concreted magazine structure with a gas-tight plotting room was constructed between each pair of guns. At one point 87 batteries were proposed, but only about 65 were built and 45 armed before construction was suspended late in World War II. Approximately 140 barbette carriages were constructed.[13] Some additional 6"/50 caliber ex-Navy guns were mounted in the year after Pearl Harbor to provide some defense while the new batteries were under construction; locations included Alaska, American Samoa, and Suriname (formerly Dutch Guiana) among others.[14] Some of the M1900 weapons on pedestal mounts were retained in service or relocated to better positions during the war, but the disappearing guns were mostly scrapped by 1944.[15] Following World War II the entire coast defense system, including almost all of the 6-inch guns, was scrapped.

• 6"/50 caliber gun - contemporary US Navy weapon, used on ships circa 1900 and as coast defense in World War II
• BL 6-inch Mk VII naval gun - contemporary British weapon, used on ships, as coast defense, and as field artillery
• 6-inch gun M1917 - British 6-inch gun Mark XIX, acquired as field artillery by the US in the World War I era; all US weapons transferred to Brazil as coast artillery 1940–41