My .260 is really set up for "practical" LR shooting, which is usually about 200 - 800 yards due to logistics, sometimes to 1000-- that's why my data card only went to 1000. I've used it with various levels of success (as much dependent on the day and conditions as the range or anything else) from 1000 to the max of 1672 as described.
For shooting very small targets past 1200 yards, a 12x scope - even a clear S&B - is not quite up to the task, and the 260 isn't in the same class past 1000 yards as 7RM, 338LM, etc.
Hey, if you've got a safe backstop and the opportunity to do so, launch some rounds way out there. You might be surprised.
-z