Jerry Pournelle

Okay, who besides David Drake for realistic MilSF by someone who served in the Military? Jerry Pournelle. He's somewhat famous for co-winning a Hugo for The Mote in God's Eye (TMiGE) with Larry Niven.

I would count TMiGE as MilSF, or enough so that I recommend it. If you like the MilSF parts of that then King David's Spaceship by Pournelle alone is also recommended.

Jerry Pournelle also has a series about future mercenaries, "Falkenberg's Legion." The more recent episodes were co-written with S. M. Stirling. They're all available in The Prince. 1000+ pages of MilSF goodness.