These webpages may be of interest to you:
http://world.guns.ru/machine/mg51-e.htm
and the free entry on Jane’s Online for the SIG 710-3
http://www.janes.com/extracts/extract/jiw/del01535.html
Jane’s Infantry Weapons gives the LMG config of a SIG 710-3 a maximum ‘effective range’ of 800 m.
AFAIK, the main users of the SIG 710-3 have been the Bolivian Army, Royal Brunei Land Force and Chilean Gendarmerie. I don't think the Swiss ever adopted SIG GPMGs. They were happy with the MG 51. SIG MGs have been out of production for at least 12 years.