The book mentioned by PetedeCoux is in line with the quality of Dieter Storz’ book on German rifles or Mathieu Willemsen’s on the collection of rifles tested by the Dutch musketry school.
The only weak point of the new book in my view is its lack of data on ammunition. See also below.
An extremely valuable addition for any research is a supplement volume, containing 33 German regulations and field manuals regarding machine guns from 1899 to the end of WW1, for example the specification regulating machine gun acceptance. In particular the small size field manuals are shown as 4 pages on a single book page, resulting in totally around 1800 manual pages shown on the 584 book pages (if I counted correctly). Beware, these are reproductions of the originals, being German language and mostly printed in Gothic fonts.
Although on the mentioned 1800 original German manual pages there is of course not a single mention of “7.92”, only many mentions of “7.9”, the authors show a severe blackout as military historians by using 7.92 in their book.