28 cm SK L/40 gun

The 28 cm SK L/40 was a German naval gun that was used in World War I and World War II by all German pre-dreadnought battleships. It was the first large German naval gun to use a horizontal sliding block breech design rather than the interrupted screw more commonly used in guns of this size; it was also the last large German naval gun to load all its propellant in a single case. It was adapted for land service after the pre-dreadnoughts began to be disarmed beginning in 1916. Four guns served on coast defense duties on the island of Wangerooge during World War I on fixed mounts. During World War II these guns were transferred to Brest. Others were used in railway mountings in both World Wars.

Description
The 28 cm SK L/40 gun weighed 45.3 t, had an overall length of 11.2 m and its bore length was 10.401 m. Although designated as 28 cm, its actual caliber was 28.3 cm. Early guns, possibly designated as the 28 cm K L/40, were built using the older hoop method of construction, but later guns used more modern built-up techniques with an inner gun tube and several outer jackets. It was the first large German naval gun to use Krupp's horizontal sliding block, or "wedge", as it is sometimes referred to, breech design rather than the interrupted screw used commonly used in heavy guns of other nations. This required that the propellant charge be loaded in a metal case (usually brass) which provides obduration – it seals the breech to prevent escape of the expanding propellant gas. The 28 cm SK L/40 was the last large German naval gun to load all its propellant in a single case; later guns required a fore charge in addition to the main charge in the cartridge case.

Naval turrets
These guns were used in twin gun turrets as the main armament of the - and pre-dreadnoughts. The Drh.L. C/01 turret was used by the Braunschweig- and Deutschland-class battleships, one twin gun turret at each end. Its guns could depress 4° and elevate 30° and could traverse about 150° on either side of the centerline. It could fire a 240 kg L/2.6 armor-piercing shell to a maximum range of 18,830 m at maximum elevation. These ships stowed 85 rounds per gun. The rate of fire for both types of turrets was about 2 rounds per minute.

Land mountings
Beginning in 1916 some guns were adapted for use ashore. One obvious change made for land service was the placement of a large counterweight just forward of the trunnions to counteract the preponderance of weight towards the breech. This, although heavy, was simpler than adding equilibrators to perform the same function. It was fitted with a hydro-pneumatic system to absorb the recoil from firing and to return the gun to its firing position, ready for the next round. The first four guns were placed in Bettungsschiessgerüst (firing platform) (BSG) mountings in 1917 for coast defense duties as part of "Batterie Graf Spee" on the island of Wangerooge. These were a semi-portable mount that could be emplaced anywhere after several weeks of labor to prepare the position. It rotated on a pivot at the front of the mount and the rear was supported by rollers resting on a semicircular rail and was sometimes equipped with a gun shield. The gun's rate of fire in these mounts was about one round per five minutes.

Other guns from the battleships were put on railroad mountings as the 28 cm SK L/40 "Bruno" and used as railway guns.

Ammunition
Before and during World War I, the gun used about 73 kg of RP C/12 (Rohr-Pulver – tube powder) propellant that was a mix of nitrocellulose, nitroglycerin and small amounts of other additives with a calorific value of 950 and an uncooled explosion temperature of 2975 kelvins. In World War II, it was replaced by RP C/38 that substituted diethylene glucol dinitrite for the nitroglycerine which had a calorific value of 810 and an uncooled explosion temperature of 2495 K. This had the virtue of being harder to ignite, lessening the risk of a catastrophic fire or explosion, and reduced the erosive effects of the gaseous propellant on the gun's bore. Sources differ on the amount of RP C/38 used by the gun during World War II; Campbell says 70 kg, but Hogg says 67 kg.

These guns mounted fired two types of shells during World War I: armor-piercing (AP) L/2.6 and high explosive (HE) L/2.9 types. During World War II, the guns fired a wider variety of shells, including high explosive L/2.9, L/4.3, and L/4.1 shells and a heavy HE L/4.4 projectile; the AP and HE rounds weighed 240 kg, while the heavy projectile weighed 284 kg.

Due to the greater elevation available in the BSG mount, the ''Sprgr L/4.4 m. Bdz. u. Kz. (mit Haube)'' had a maximum range of 27,750 m.

Armor penetration
One source credits the Pzgr L/2.6 shell with the ability to penetrate 160 mm of side armor at 12,000 m.

Naval guns
The Treaty of Versailles allowed the Germans to retain four pre-dreadnoughts, although only two, SMS Schleswig-Holstein and SMS Schlesien, were rearmed with their original 28 cm SK L/40 guns. The former fired the first shots of World War II when she began bombarding Polish defenses on the Westerplatte on 1 September 1939 while the latter also participated in the Polish Campaign. However both ships were relegated to training duties shortly afterwards.

Coast defense guns
Some of the 28 cm SK L/40 guns were transferred to the German Army from the Navy (Kaiserliche Marine) when the pre-dreadnoughts began to be disarmed and relegated to training duties in 1916 after the Battle of Jutland had proved that they were not suitable for contemporary naval combat. The first four guns, formerly used on SMS Lothringen, were placed in BSG mountings in 1917 for coast defense duty as part of "Batterie Graf Spee" on the island of Wangerooge.

During World War II only seven guns were used on as coast defense duties on BSG mountings. The four guns of "Batterie Graf Spee" had survived World War I and were transferred to Brest, France in 1940.

Railway guns
Approximately twenty other guns from the battleships were put on railroad mountings as the 28 cm SK L/40 "Bruno" and used as railway guns. A number were kept by the Kaiserliche Marine and used on coastal defense duties, but the others were used by the Heer in more traditional roles as long-range heavy artillery. Surviving weapons were used by the Germans in World War II as coast defense duties.