Since 1935, the USSR had been developing new battleships as part of the "Big Fleet" program. The "Project 23" battleship was intended to surpass any existing or prospective foreign battleship. Its main caliber artillery was to consist of nine 406-millimeter guns—the largest caliber achieved in naval artillery worldwide at that time (the main caliber of battleships in the Soviet Navy was 305 millimeters). The long range of the 406-millimeter guns was achieved by extending the barrel length to 50 calibers and increasing the charge. The projectile length, to enhance destructive power, reached 4.7–5 calibers.

By the decree of the Council of Labor and Defense under the USSR Council of People's Commissars dated July 16, 1936, the development of the 406-millimeter gun was assigned to the Leningrad "Bolshevik" plant. The gun was designated B-37. Barrel durability was to be ensured by two methods—either by shrink-fitting or by using a liner.

The liner was developed at Research Institute No. 13, and the cradle with recoil mechanisms was designed at the Design Bureau of the Leningrad Mechanical Plant named after Stalin under the leadership of Tolochkov. At the same plant, under Bryl’s leadership, the three-gun turret MK-1 for the battleship was designed, and under Ponomarev’s leadership, the MP-10 test mount ("polygon stand") for gun trials was developed. Ammunition development was carried out by the Leningrad branch of Research Institute No. 24, Central Design Bureau No. 22, and Research Institute No. 6 of the People's Commissariat of Armaments.

At the beginning of 1938, the technical project for the 406/50-millimeter B-37 gun was approved. Between 1939 and 1941, the "Barricades" plant in Stalingrad produced twelve 406-millimeter barrels, eleven of which were lined. Of the four "Project 23" battleships laid down between 1938 and 1940—"Soviet Union," "Soviet Ukraine," "Soviet Russia," and "Soviet Belarus"—none were completed. However, the MP-10 test mount, manufactured by the Novokramatorsk Machine-Building Plant named after Stalin, was commissioned in August 1940 at the Scientific-Testing Naval Artillery Range (Rzhevka), with a 406-millimeter barrel installed on it.

The B-37 gun barrel in its first version was shrink-fitted with four cylinders and a jacket; the second version was equipped with a replaceable liner. The breech was attached to the barrel with studs and a thrust ring—this was the first time such a design was used in domestic artillery technology. The two-stroke piston breech block had a three-step rifling, opened upwards, and was equipped with a pneumatic balancing mechanism. The rotation, unlocking, and locking of the breech were performed using electric motors.

The gun was designed for separate cartridge loading with a galvanic-percussion firing mechanism. The ammunition was to include armor-piercing, semi-armor-piercing, and high-explosive shells. The powder charge could be combat (enhanced, 310–320 kilograms) or reduced. Ignition means included galvanic and percussion primers. There were reports of developing long-range shells with firing ranges up to 73 and even 99 kilometers.

The foundation for the MP-10 mount was a reinforced concrete block weighing 720 tons. A cast steel ring with a ball race containing 96 balls was mounted on it. The rotating part, shaped as an enclosed structure with a gun cradle, rested on the ball race with a diameter of 7.46 meters. The length of the mount without the gun was 13.2 meters, and the height from the plane of the ball race was 5.8 meters. Shells and half-charges were delivered by cargo truck, unloaded onto the mount’s loading table, and then transferred to the loading tray located in line with the barrel bore axis. Loading into the breech was done by a chain-type rammer developed for the MK-1 turret mount. In 1940, the shrink-fitted barrel was tested on the MP-10 mount. It was decided to put the lined barrel into serial production. This barrel was installed on the MP-10 mount on the eve of the war. At the start of the war, the mount was given full 360-degree rotation.

The 406-millimeter mount, together with one 356-millimeter and two 305-millimeter guns, was included in Battery No. 1 of the range under the command of military technician 2nd rank Kukharchuk. On August 29, 1941, the 406-millimeter mount fired its first shots at concentrations of enemy infantry and tanks that had broken through near the "Krasny Bor" state farm (Kolpino direction).

The B-37 gun on the MP-10 mount successfully shelled German and Finnish troops on the Kolpino, Nevsky, Uritsko-Pushkin, Krasnoselsky, and Karelian fronts. In 1942, Leningrad industry restored production of 406-millimeter ammunition specifically for it.

Fire from the 406-millimeter gun was corrected by posts of Baltic sailors—one of the fleet-wide correction posts was equipped at the slipway of the Zhdanov Shipbuilding Plant.

During Operation "Iskra" to break the siege of Leningrad in January 1943, 33 406-millimeter shells were fired at the enemy. One hit destroyed a concrete structure of the 8th State District Power Station, used by the Nazis as a fortification. Nearby, a crater from another shell was found, 12 meters in diameter and 3 meters deep. It is no surprise that hits from 406-mm shells had a strong psychological effect on the enemy. The 406-millimeter gun also destroyed target No. 23 (height 112.0)—a strongpoint of enemy resistance on the approaches to the city of Pushkin from the north. In total, during the siege of Leningrad, the 406-millimeter gun fired 81 rounds.

After the war, the MP-10 test mount was used for testing other large-caliber systems and their ammunition. For example, in 1957, the largest domestic experimental gun—a 650-millimeter smoothbore 6R-105 designed for testing armor-piercing aerial bombs by firing—was mounted on it. During the same period, mock-ups of 406-millimeter special (nuclear) shells were also tested.

Sources:

https://ru.wikipedia.org/wiki/406-mm_naval_gun_B-37

https://warfor.me/406-mm-pushka-b-37/