It requires some creativity to turn "turning radius" into "wearing diameter", particularly since it applies to the machine gun, and not to the vehicle on which it is mounted.
I wonder if this is not a typo for "bearing diameter", which applies to the turret holding the machine gun on armored vehicle. But of course there is a problem with 8,5 m (with a comma in English???).
CF this note on the entry for 3infantry tank" in Wikipedie :
"The turret ring bearing must carry the recoil force of the main gun, reduced by the mounting's buffer.
The turret and bearing diameter controls the bearing capacity, thus the limit of armament capacity.
http://en.wikipedia.org/wiki/Infantry_tank
The translation workplace 
