I have a 3.5-10x40 on my 416wby for out to around 1000yds. I consider it OK to about 500yds on 10x for target work and fairly difficult beyond 700yds. To identify and hold a consistant point of aim becomes the problem with normal size targets. How close can you hold to your aimpint at 1000yds with just 10X?
I think 10x is minimum for deer at the stated 600-1000yd range. I hunt moose and think it's minimum for them but it sees close range work also.....bear in the bush.