Post-World War I planning for the Marine Nationale included discussions for a 40,000 tonne battleship armed with the largest possible main guns. This size was meant to be comparable with British designs (HMS Hood at 42,760 tons) and USA designs (USS South Dakota at 43,000 tons). The size of the main guns was derived from the lessons of Jutland (Skagerrak) which were stated as follows in an October 1919 report: 1) The British CPC shell had insufficient penetrating power and broke up on armor plating. This shell type was rejected for new designs and instead were to be APC types. 2) The APC shells favored by the Germans had good penetration but had too small a burster. This design was attributed to the smaller caliber of the largest German guns (30.5 cm) which forced the Germans to concentrated on penetration at the expense of burster size. The conclusions of the report was that the French should develop the largest caliber gun possible which would fire an APC shell to long range and still have good penetration power.

Some members of the commission wanted to develop a 40 cm (16") gun first to reduce risk, but this was rejected as it was felt that trials with a 45 cm gun would be applicable to a 40 cm gun but the reverse would not be true. It was also pointed out that foreign navies (the Japanese and Americans) had already started work on ships with 40 cm guns and would therefore be planning larger calibers by the time the French battleships were laid down.

Development of the 45 cm gun was started in 1920 and preliminary drawings of the gun and its APC projectile as well as the quarter propellant charges were completed in that year. It is not known if actual design work and manufacture of a prototype was started at the Ruelle artillery facility. The plans for it were not formally approved until April 1921 and it would have probably taken two to three years for a cannon of this size to be made. Funding had not been authorized by the time the Washington Naval Conference convened in November 1921. Even if work had started it would have been quickly cancelled as preliminary discussions in Washington showed that the size of these ships and guns were to be prohibited in the final treaty.

French guns designed between about 1902 and 1922 were designated in centimeters.

• ^This weight included the breech mechanism and the guides (slides).
• ^Rifling was to be angled at 7 degrees.

• ^Apparently, only an APC round was designed.
• ^The design of the APC M1920 generally resembled the 203 mm shell fired by the Treaty Cruisers. It had a long, tapered windscreen, a bourrelet, two driving bands and a flat projecting base plug. The AP cap had a pointed design similar to the later 330 mm Mle 1935 and 380 mm Mle 1936 APC projectiles.
• ^

  Actual designations for Ship Projectiles

  APC Model 1920	450 mm OPf Mle l920

• ^The propellant charge was in quarters, making them unusually heavy.
• ^The high muzzle velocity and working pressure would have resulted in long range but a short barrel life.

"French Battleships of World War One" by John Jordan and Philippe Caresse

08 January 2023 - New datapage