Manuals are supplied with a new resistance welder for the major components of the machine. If this is an old in house machine or a used machine manuals may not be present. In that case a thorough search of the machine to find the machine builder label with model number and serial number (if present) is in order for all of the components.
A weld gun would have the same information on it's components.
With as much of this as possible in hand your options for manuals are first to contact your local distributor. They may be able to help.
If not the next and best source would be the Resistance Welding Manufacturing Alliance – RWMA @ www.rwma.org
RWMA has a member directory of resistance welding manufacturers from all over the world one of whom may have the information you seek.
Reference: RWMA Membership Directory