Good luck finding 500+ man, i've been looking for several years too.... all over the Roosevelt national forest. I've found a couple places with 125ish meter lengths and one with 290ish , but the 290 is across the gap of a small canyon, so placing and retrieving targets is a huge pain in the butt.

I usually just take advantage of hunter sight in dates at a few private ranges around the area.

You could look in to the Pawnee Grasslands, but i've never shot there.