My favorite varmint round is the 220 Swift. I do not have any shots as long as 700 (longest about 565 yds) but I think it could easily get there even with the 55gr bullets I am running through it. I do plan on on rebarreling with a quicker twist when the time comes to help it reach out a little further.
As soon as I receive the rifle, I will be using a rifle called a 300 Varminter which is based off a tight neck .337 Match 300 WSM Chamber. The bullet I will be using will be the 125grn BT and 110grn -Max at a velocity of 4000+fps. The rifle is being built for varminting out to 800 yards.
I use the 204,223 and 243 with light bullets for most of my doggin but when the yardages get past 500 the 6 & 6.5 284s come out to play with 105 and 140 bullets. The 75Amax out of the 22-250 does alright out there too;)
The wind here mandates a heavy bullet if your gonna getem past 500!
__________________ "Make everything as simple as possible, but not simpler." -Albert Einstein (1879-1955)