The standard anti-aircraft defensive weapon of early-war type VII U-boats: http://navweaps.com/Weapons/WNGER_20mm-65_c30.htm http://www.steelnavy.com/WEM400GermanAA.htm http://www.uboat.net/technical/flak.htm
Starting with the WEM PE as a base which acted as a template for the main body of the gun, I scratchbuilt the rest of the gun with 0.13mm styrene, stretched sprue (barrel, top of the gun’s main body and “phi-shaped” sight mount, barely visible in the pictures), 0.13mm copper wire and pieces of super-thin 1/700 PE railings from L’Arsenal. The 21 (!) pieces were assembled mainly w CA and there are also two micro-welds. I built the gun in 2011. I had decided to start my project with the most difficult parts of the kit (the 2cm and 8.8 gun) and to date am still stuck with them(!). I am sorry for I didn’t take WIP shots of the gun at the time as I hadn’t thought I would keep a photographic diary of my project back then. Enjoy the final result.
Below, “The Three Amigos” (from L to R): WEM PE, my AA gun and the one by Mirage Hobby.
As you probably noticed I omitted the muzzle brake since I think that the small scale (or should I say my dexterity?) doesn’t allow to reproduce it in a convincing way without hampering the final result. Fortunately there’s photographic evidence of operational use of C/30s without the muzzle brake. The picture below shows an MG C/30 operating without it: I am happy and in peace with my decision 🙂 .
Hocus, Pocus, Fidibus, Three Black Cats, aka my 1/400-scale tribute to U-48 
My congratulations for your excellent work has served me a lot since I am putting together a German battleship Admiral Graf Spee,
I am Uruguayan and in my country you cannot get books or literature on the subject
Duilio thank you for your kind message, am sorry for the belated reply. Since I seldom receive comments, I don’t usually check for them and noticed yours just now (the first comment in many years: thank you for that!). My apologies. I am sorry for updating the blog so rarely but it depends on my very slow building rate (I ‘ve just updated another section today after one year, that’s embarassing!).
I am glad that my work helped you, this is rewarding for me: it’s what sharing is meant for. Thanks for dropping by and Good Luck (“in bocca al lupo/crepi il lupo”, as we say in Italy) with your Graf Spee project!
SG