At the link below is a photo of the breach end of a field gun with a camoflage pattern outlined. The gun looks about 3-inch or 75-mm size and reminds me of a Krupp. I think I have other photos of front of this this same gun, or at least the same camoflage pattern, at the Brussels museum.
Piece of cake! That is a German 15cm sFH13 Lang. It was one of the most important heavy guns in the German Army: WW1 was the war when it was discovered that a calibre around 15cm is pretty much optimal when it comes to range, versatilty and effect. (There is an article on it to be found in the Artillery secion of thsi site.) The photo is taken in the Army Museum in Brussels, who have a amazing WW1 section.