We don't really "Zero" our weapons at one range, we consider it constantly fluid and changing. We have a BDC (bullet drop compensator) on our scopes for our elevation knob. We estimate the range to the target, dial in the range on the BDC along with our guessimate adjustments (for humidity, temperature, etc) and then engage (of course, wind adjustments are dialed in also). So, after you do the initial zero of the rifle at a given range, and then slip the BDC, we never really have a zero, so much as just verifying the BDC at each range when we can. I like to carry my rifle with the BDC set at 600m, and then adjust if/when needed for the target.
I agree its experience. But remember that military snipers have shot hundreds if not thousands of rounds through their individual rifle at various known ranges, noted all of the required elevation and windage changes in their data book, and have created an effective external balistics table for their weapon for just that reason. When you can't shoot spotters, you need to "know" what to expect from your ammo and rifle under certain ranges and atmospheric conditions. An accurate BDC certainly speeds up your ability to get the first shot off quickly, but you can do it without the BDC if you keep a good data book.
That is also where the mil-dot riflescopes come in. If you have a thorough understanding of the ballistics of your rifle and load, the kind that only comes with many hours of practice, you will be able to judge where your bullet will impact before you pull the trigger. Of course you will have to combine this with a reliable rangefinder.
well, remember, the mil-dots PRIMARY use is range finding, so just use your mil-dots as your range finder. Of course LSR's are nice, if you have a powerful one with a good target to give a good reading.