Well, first of all the scope+base combo needs to allow it.
For example, a Barrett M82A1 has a slanted 27 MOA rail. Add 30 MOA LaRue rings, and a 4.5-14x50mm Leupold scope with 70 MOA elevation, and you have...
127 MOA total. This should provide enough elevation for shooting at nearly 2,000 meters with M33 ammunition.
However, in ArmA 3, zeroing works differently. I will use the GM6 Lynx as an example, firing normal (B32 API) ammunition.
In-game, the scope on the rifle "zeroes" to 2500m. In-game, it requires about 196 MOA to reach that!
However, take the scope off the Gepard, and put it on the 7.62mm EBR. You can still zero all the way to 2500m! To hit at 2500m, the 7.62mm would require over 600 MOA elevation! In real life, no rifle scope exists that even comes close, but in ArmA 3, it is instantly possible. Unrealistic.
Now, consider with high-velocity APDS ammunition: the Gepard only requires about 74 MOA to reach 2500m. Completely different, and totally capable for many scopes.
Yet it has been the same scope the entire time... so ArmA 3's system of zeroing presents serious realism problems.
Now, imagine if the scope didn't zero to "2500m". Instead, if it used angles, in MOA, or mils.
If you took the scope off the Gepard, and placed it on the 7.62mm, that particular realism problem (scope being capable of 600 MOA elevation) would not exist. Instead the 7.62mm would just fall short, being launched at the same angle the .50 was. Etc.
The only problem with this system is that people would have to learn MOA elevation for different ranges and calibers, and since people tend to be lazy about these things, most gamers would definitely not like that.
If you had a scope zeroed at 200m on a .45 ACP (silly, just an example) and put it on a 7.62 right now, the 7.62 would hit at 200m.
But if you had a scope set to 59 MOA (in-game required elevation for .45 ACP at 200m) and put it on a 7.62, it would hit at about 1100m.
So the point is, it would be more realistic to have scopes use angles, rather than ranges. Although this won't happen for gameplay purposes.
In respect to the suggestion, I think 2500m is far enough, why go to 2800m. Besides, the .408 only goes 2,650m, the Lynx goes much less than that without APDS. The bullets simply disappear in thin air.
And who makes 2500m shots anyways? Especially in battle scenarios. These are typically only editor test shots. I had only ONE shot at over 2400m on an actual enemy player in ArmA 2, and I have made *lots* of shots at enemy players, so this is super rare even to get the opportunity.
I think adding zeroing to 2300m, 2500m has changed people's expectations about long-distance shots. :/