Not sure about milk jugs, but I have scored in the 590's/600 points in Mid Range Prone with a match AR15 and Service Rifle AR15 at 600 yards. Shooting Prone with sling. The 10 ring is 12 inches, the X is 6 inches. With that said, I think with a bolt rifle with bipod or a bag, I could do pretty good on jugs. The thing about High Power competition, if you make a bad wind call, the black bull is large enough for you to see your mistake when the man in the pit marks it with a spotter, then make corrections and get back in the 10 ring. With a jug, only thing you have is flying dirt, if you miss, if even that.
The club I'm a member of has steel out to 840, but it's all larger than a milk jug.