7mm WSM, or smaller. If you're spotting your own shots, smaller is better. The 7mm WSM is being discovered as a real killer 1k cartridge, and I believe it holds the current record. I'd also think long and hard about a 308 or better still a 7mm08 or 260, they'll all do 1k with a minumum of fuss
And while the 6.5-284 is good, IMHO the 284 alone is better - lobbing a 160-ish grain high BC bullet up in the .600 range at 2700fps or so would be just the ticket. And barrel life ought to be a lot better than the 6.5-284