I have a book which says the I-16 tip 24 was a four machine gun plane, and the I-16 tip 27 had two SHVAK and two machine guns.
The book is "The complete book of fighters", it's heavy but seems to be comprehensive. I presume there are good sources for their data.
It was remaindered (sold off cheap) when I got it, so it probably isn't available new, it seems to be available second hand via Amazon:
http://www.amazon.co.uk/Complete-Boo...7790613&sr=1-1