They still have 600 yard military rifle matches - not necessarily associated with the military, but the shooters use military rifles such as M-14, M-16, Garand, etc. My friend in Ohio has competed in this.
They use open sights, and IIRC, shoot at those distances from the prone position. I’ve shot 200 yard matches - we shot standing, kneeling, and prone.
Just curious when you shot - I have competed at Camp Perry several times.