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The 15 cm Mantel Ring Kanone L/30 was a 15 cm 30 caliber long Krupp Mantel Ring Kanone (MRK). It was a rifled breech loader built-up gun with a Krupp cylindroprismatic sliding breech. Its 30 caliber long predecessor of 15.24 cm / 6 inch caliber was designed as the first 15&nbap;cm 30 caliber Krupp gun to profit from a slower burning gunpowder invented in the early 1870s.

Context
In the race of ship armor against ship artillery, the primary course of action of the artillery manufacturers was to increase the caliber of their guns. As a second option, they attempted to increase the velocity to which the projectiles were propelled. This boiled down to increasing the charge of guns. There were several reasons why significantly increasing the charge of existing guns was very difficult.

One of the main problems that had to be overcome to increase the effectiveness of guns, was that an increase of the charge leads to an increase of the peak atmospheric pressure inside a gun. In other words, simply increasing charge of an existing gun leads to increased wear, and sooner or later, the bursting of the gun barrel. The first series of Krupp Ring Kanone overcame this problem by using the so-called Prismatic Pulver c/68 (P.P. c/68), a kind of gun powder that burned slower than traditional gunpowder. This is also called less-offensive.

In the 1870s, manufacturing a less-offensive gunpowder could be done. The real challenge was in quality control. Gunpowder batches from the same manufacturer and having the same specifications could lead to differences in average atmospheric pressure of 200-300 Atm. This made it very hard to develop a reliable less offensive kind of gunpowder. In January 1872 the management of the Spandau gunpowder factory started to experiment with a kind of gunpowder that would finally lead to the even less offensive P.P. c/75 which had a density of 1.75 and only one channel. In October 1875, the Allgemeines Kriegs Departement approved its use under the P.P. c/75 designation.

The last 15 cm L/26
By 1875 the latest Krupp innovations were the Mantel Ring Kanone construction (in e.g. the 15 cm MRK L/22), copper driving bands, the parallel grooves, and a rifled chamber. When the new P.P. C/75 type of less offensive gunpowder had been developed and approved, it could be taken as a base for developing new guns. The logical course of action would be to make a longer 15 cm Mantel Ring Kanone (MRK) for a higher charge. The existing 15 cm RK L/26 had been tested for optimal rifling up till Summer 1874. It probably served as a base for the next design.

In July 1878, a 15 cm MRK L/26 was tested at the Krupp artillery range in Meppen. That is, one can assume that it was a Mantel Ring Kanone (hooped & jacketed). The main differences with the 15 cm RK L/26 were the 36 parallel grooves with 3.5 mm wide lands, and a progressive twist length of 6.71 m.

The projectiles fired by the 15 cm MRK L/26 were somewhat heavier than those in use with the RK, and the charge was 9.5 kg of roughly granulated powder of 13-16 mm or 8.5 kg of prismatic powder with 7 channels and a density of 1.64. The other guns at this test (35.5 cm MRK L/25, 30.5 cm MRK L/25, and 28 cm MRK L/22) used gunpowder with a density of 1.75 and one channel, which conforms to P.P. C/75. The velocity measurement failed. However, in a May 1878 test it was 500-514 m/s.

The July 1878 test showed that Krupp lengthened its heavy 35.5 and 30.5 cm guns before it lengthened the 15 cm. This seems strange and might have had to do with Krupp's attempt to create a 15 cm armored gun, the so-called Panzerkanone. This was tested in Bredelar in 1877 and in July 1878, when it had a caliber of 15.5 cm.

The 15 cm MRK L/28
Designing a 15 cm MRK for a heavier charge sounds easier than it was. If the charge of the gun was to be higher, the chamber of the gun had to be lengthened to give the gunpowder the same relative space to expand. Therefore increasing the charge by itself would increase the length of the gun. If the charge was increased by using less offensive gunpowder, then it was very likely that the projectile would leave the barrel before all the gunpowder has exploded. Therefore, the barrel had to be lengthened on both sides to fully profit from less offensive gunpowder.

When the July 1878 tests took place, the regular length of projectiles was 2.8 or at most 3 calibers long. It was clear that longer, i.e. heavier projectiles would have more punch than shorter ones. Longer projectiles would also loose less velocity due to drag than lighter ones. However, longer projectiles were prone to be misformed in the barrel, to have an unsure flight, or to hit at a wrong angle. Longer projectiles also required a stronger charge, a faster twist rate, and a stronger carriage. The latter because of the higher and or longer recoil. Taken together this meant that effective use of longer projectiles required a totally new design.

In December 1878, Krupp had a 15 cm L/28 gun ready. It weighed 3,960 kg and had a progressive twist rate of 25 calibers or 3.73 m (giving the projectile a faster twist). In January 1879, it fired a 4 caliber long 50 kg grenade with the use of charge of 15.5 kg of gunpowder of 1.75 density. Velocity was about 510 m/s, but if the older lighter grenades were used this could become 630 m/s.

During 1879, the 15 cm MRK L/28 proved the theory that longer projectiles were very useful, expecially because these did not loose their punch at a distance. In August 1879, the 15 cm L/28 was tested in Meppen. With strong winds of 7.64 m/s the longer projectiles proved to remain accurate. The gun fired an armor piercing grenade of 51 kg at a velocity of 509 m/s and a regular grenade of 40 kg at 559 m/s. At 1,979 m from the muzzle the former had a velocity of 395 m/s, the latter had 396 m/s.

The 15.24 cm MRK L/30
Based on the results of tests with smaller guns, the 15 cm L/28 and the 25 caliber long guns of larger calibers, Krupp then designed a whole new system of 30 and 35 caliber long guns, the so called Construction 1880 or c/80. The 15 cm gun would have a caliber pf 14.91 cm and would come in a 30 caliber long and a 35 caliber long version.

However the first L/30 gun that Krupp made had a caliber of 152.4 mm (i.e. 6"). Its consistent name could have been 15 cm MRK L/30. There were several possible grounds why Krupp first made this L/30 gun: The 15 cm caliber was in general use on ships and as a coastal gun; It was only a small step from the L/28; and a 15 cm L/30 could be made easier and with less cost than a heavier gun. However due to its odd caliber, the general suspicion was that Krupp wanted to provide clear prove that it could make a better gun than the new British 6 inch gun. However, these suspicions are based on the assumption that the construction of the 15.24 cm MRK L/30 followed from the Construction 1880 plan.

The new 15 cm MRK L/30 had a caliber of 152.4 mm (i.e. 6"). It was 4,600 mm long (L/30) with a length of bore of 4,180 mm (L/27.5). The chamber was rifled. It had 36 parallel grooves with a progressive twist rate of 3.81 m or L/25. Its weight including the breech was 4,200 kg.

In July 1880, this gun was tested in Meppen. It fired a 50 kg grenade with a velocity of about 520 m/s. A 35 kg grenade was fired with a velocity of about 615 m/s.

The 15 cm MRK L/35
In March 1882, Krupp tested a 35 caliber long 15 cm MRK L/35 in Meppen. It fired L/3.35 long armor piercing grenades and 4 caliber long explosive grenades. Each weighed 51 kg. The charge was was 17 kg of gunpowder with a density of 1.75 and a single channel. Velocity was about 535 m/s. The 15 cm MRK L/35 again had the regular Krupp caliber of 14.91 cm.

Carriage
The 15 cm MRK L/30 was mounted on the 15 cm Rahmen Lafette C/83 and the 15 cm Mittel Pivot Lafette C/83.

Projectiles
The 15 cm MRK L/30 used the 15 cm Granate L/4, and the Stahlgranate L/3.5. Both were shot with a charge of 15 kg of Prismatic Pulver C/75. These had a conic driving band without indentation and a centration wart.

Use
In 1885, the 15 cm MRK L/30 was known as 15 cm Kanone L/30, abbreviated 15 cm K. L/30. At the time, the Mantel Ring Kanone construction was the latest. Therefore, all Mantel Ring Kanone were simply called 'Kanone'. The older Ring Kanone were called 'Ring Kanone'. About 8 years later, something similar happened. When the quick-firing Mantel Ring Kanone or 'Schnellade Kanone' were introduced, the newer guns received the 'Schnellade Kanone' or 'SK' label, while the older ones retained the 'Kanone' label.

In 1897 the Arcona-class corvettes Arcona and Alexandrine had 15 cm Mantel Ring Kanonen L/30, which they already had in 1879.