There is also a short passage about an encounter with UB 10 including use of the machine gun (but no details of it) on the pages 177-182 in Werner v. Langsdorff (ed.), "U-Boote am Feind", 1937. I have found a scanned PDF of that book in one of the online libraries but, unfortunately, I have not recorded where.
In that book is also the only interior picture (torpedo room of UB 16) of a UB I class uboat that I have ever found.

Cheers,
Anders