http://jamesdjulia.com/item/3738-386/

With the advent of aircraft, marksmanship instruction gained a huge new element of complexity. Now there were gunners firing at rapidly moving aerial targets from the ground and worse, gunners in moving aircraft shooting at other moving aircraft! Classic shotgun sports were often used to train gunners to lead flying targets, but that wasn't deemed sufficient by all countries. What was desired was a way for an instructor to actually see a student's sight picture while firing.

The solution to this need was to build cameras which duplicated the handling characteristics of aircraft machine guns. The gun camera we are looking at today is an MGK-1000, which is made to simulate an MG-15 Luftwaffe machine gun. It is a movie camera, and each frame of film has a crosshair superimposed over it as well as an image of the vane sight's position at the time the frame was exposed. This allows an instructor to see a record of exactly where the student was "shooting" and determine what corrections need to be made.

In addition to the camera functions, the MGK-1000 has elements intended to duplicate ancillary machine gun functions. For example, the spring that powers the camera is contained in a magazine body, and is weighted to duplicate the weight of a fully loaded magazine. The spring allows the camera to run as long as a 75-round drum would take to fire, at which point the trainee must swap it for a new "magazine" to continue "firing".