Using optics on a non-living target it would have to be 1200 yards or so with an AR-50 .50 BMG topped with an 8-32x56 Nightforce sending 750 gr Barnes LRS downrange at about 2700 fps. I had one of those bright plastic sleds standing about 48 inches tall tied to a small tree down a powerline road. The optic was newly mounted and not sighted in at any particular range and I didn't have a rangefinder that went out that far, so I used the MOA-based stadia lines to compute range at about 1200 yards or a little farther. It took about 6 rounds of firing, seeing the hit, adjusting, and firing before I was able to get on target but after that I put three in a row on the target with little difficulty.
With iron sights, it would be the M240 and M2 out to about 1000 yards, if you want to count that. We were shooting old deuce and a halves and 55 gallons drums, which was fun, but not really all that difficult, esp with the M240. I did better with it than with the M2. Other than that, I have shot and hit out to 600 yards with the factory irons on my M1A and qualified 'expert' out to 500 yards with the irons on an M16A2.
On a living target, I've hit out to about 300 yards with my Leupold equipped M700 7mm Rem Mag.