Merc is a military role-playing system in which the players assume the roles of modern mercenaries engaged in counter-insurgency.
[1] The rules cover character creation, specialties (e.g. map expert, medic, escape artist, etc.
[1] Two years later, in 1983, FGU published Merc Supplement 1: In Country, which provided more rules for things like vehicles and airborne operations.
As it is, though, I would warn the buyer to 'approach with caution' unless he is already quite familiar with the subject matter, in order to fill in the numerous holes.
"[2] In Imagine, Paul Cockburn thought the audience for Merc was limited, saying, "Merc is clearly designed for the gun nut, the sort of role-player who likes to know just how much of a mess his assault rifle will make of a 'soft' target [...] The book is dedicated to 'Mad' Mike Hoare, (Mercenary Extraordinaire) - and I'm sure he'll be delighted.