I use a 4-16x on my .204 AR and just installed a 4.5-14x, with a Varmint Hunter reticle, on my CZ .204 that is supposed to be good for 600 yards, for a shoot coming up...as mirage gets pretty bad at times when you get over 18x where we go..
That being said, I also have an 8-32x on each of my Rem 700 (.223) and my Walther (.22lr), but don't expect to be using them on full power very often...
I very seldom make many attempts past 400 yards, unless I have a really good spotter working with me..as the wind/breeze usually costs me too many extra shots...