German Observer's MGs

[Mogadeet]

I know that the Bergmann MG-15 was mounted on some World War I German 2-seaters, but it was never a general issue. On paper (Notably a 3- volume work called Flying Guns) the Bergmann looks superb. But I can find practically nothing that provides a comparison of the Bergmann and the Parabellum and explains why the Parabellum became common, and the Bergmann not.

I also need to know when they stopped trying to mount the Bergmann.

I have a book, Fighter Aircraft of the 1914-1918 War, by W.M. Lamberton, (I have an autographed copy, signed by Lamberton,Carrick, Yeoman, Cheesman and Russel, Yehaa, yehaaa!), and all they say is "During 1916 the Bergmann gun was tried out as an observer's weapons, but it proved less efficient than the Parabellum."

Can anyone help me here?

1. What was the problem with the Bergmann?
2. Is there a resource on the Web with this information?

Thanks Bunches!

Mogadeet