FiveCore is the generic skirmish system that is also the basis for Five Men in Normandy. Both games are designed by Ivan Sorensen, creator of the free Fast and Dirty sci-fi rules. He also maintains a blog about his games, where he discusses their development and offers new optional rules.
The 11-page Tactical Primer supplement adds an army generator, campaign generator, and optional rules for stealth, explosives, snipers, and civilians.
I still need to play some battles with these rules but so far I like what I see. The "action roll" system reminds me of a similar rule in the Sacre Bleu! game, creating some unpredictable situations. There is a clever system for shooting that incorporates damage and suppression in a roll. The rules for reaction fire are quite simple. On the other hand, there is no distinction between soldiers except for weapons and possibly their skill: no training or experience attribute, for instance.
My impression is that FiveCore will shine in scenario-based games, where each side has a specific objective that they try to accomplish. The lack of in-game details of characters might be a way to push players towards narrative interpretation. Since some weapons allow shooting across the table, having plenty of cover might be necessary. Anyway, these are my expectations, and I will write more about this game after I have played a few battles.