We've used the cheapo 8" type from Aliexpress with a 223 at 200 yds, with 73, 75, and 80 grain bullets, and they're fairly easy to see through an 8-32 scope if there's no mirage..
I plan to try them at 300 when it stops raining, but I think 600 would be pushing it. Might be OK with a larger calibre and a big birding scope, 80x?
Pete