Like most other countries, Germany entered the Great War lacking in heavy long range fire capacity. There were 15cm long range guns available in Germany, but these were used on ships or in coastal defense batteries. Also, before the war Krupp had developed a 15cm flat trajectory gun, but it had been rejected by the Army, as being far too heavy to have good mobility. Soon the Army had to reconsider, and in 1914 they ordered Krupp and Rheinmetall to design such a 15cm gun, one prototype each. It is a good proof of the desperation felt, that the Army, after a short trial, issued orders of production of both designs to both companies.
Due to the weight the Krupp gun was transported in two loads. It was designed to be towed by motor transport, for instance the Krupp-Daimler Artillerie-Kraftzugmachine - see one of the photos below. Lack of Tractors sometimes led to them sometimes being towed by horse. The gun had a calibre length of 42.7, a elevation of -3° to +42°, and a traverse of 8°. The weight of the gun deployed was 10140kg. It could shoot a 52.5kg shell up to a maximum range of 22.8km. (The muzzle velocity was 749 m/sec.)
The Krupp gun was used in front-line service from early 1917. After the war some guns were supplied to Belgium, as a part of the war reparations. It was used in Germany during the inter-war years, and also in the opening stages of WW2.
The photos below - kindly supplied by Mark Hansen - show a surviving Kanone M.16 Krupp (there are about 6 surviving M16s). It can be seen in the Australian War Museum in Canberra.
The plans below comes courtesy of Ken Musgrave, he holds the copyright to it, and any commercial use must first be cleared with him:
There is no kit of this gun available, so you will have to scratch build it.