This big, heavy tool kit is perfect for those people building MG42 receivers.  These tool kits were used by the German army.  When a MG42 receiver was damaged, or thought to be out of spec, these tools were used to find the problem, align and straiten out the receiver.  I'm not a gun smith so I can't give you exact directions on how it works. However this was a very important tool kit for German armories.  I saw one of these for the first time in 1999.  After six years of searching I finally have some for sale.  Each tool kit weighs over 60 pounds and comes as pictured, used but in perfect working order $320.

If you look at the below picture on the left.  You see the two tall tubular shaped items on each side of the tool kit.  The one on the left has guides that lock into MG42 receiver rails.  Once attached the MG42 receiver turns easily on ball bearings.  The telescoping tool on the right extends to the top of the MG receiver.  It measures each part of the receiver to finds the problem areas.  The two steel mandrills in the center attach to both ends of the receiver to straiten things out.