In addition to the info Kato supplied, it's worth remembering that many of the sniper shots taken at those longer distances were done with different ammunition to the M21.
Specifically, it was either .338 Lapua or .50BMG ammo, both offer a heavier projectile than the 7.62mmN round and an increase in propellant charge.
They're less affected by wind compared to the 7.62mmN and obviously have a longer trajectory (meaning bullet drop kicks in at it's worst, further down range than it does for the 7.62mmN).
For example (using Hornady's ballistics chart for their own ammo): -
At 300m, the projectile will take a trajectory that will place it x centimetres below the line of sight (to the target).
For .308 with 178grain projectile, trajectory is -47.1cm relative to line of sight
For .338 with 285grain projectile, trajectory is -38.5cm relative to line of sight
For .50 with 750grain projectile, trajectory is -33.9cm relative to line of sight
That is to say, at 300m all these projectiles will be travelling below the line of sight (LOS) but the .338 will be about 9cm closer to the LOS than the .308 and the .50 will be about 13cm closer to the LOS than the .308.
What does all that mean? As a gross simplification, if the shooter was aiming for a target at 300m, they will have to aim up (i.e. high) 47.1 cm to hit where they are looking at when using the .308 but only 33.9 cm high if they were using the .50BMG round.
Does that make sense? It's all clear in my head but I don't know if I explain it very well!
Notes:
1. Ballistics info for all rounds was taken from a 24 inch (60.9cm) barrel.
2. .308 is close enough for this purpose to 7.62mmN. I had to use .308 as the ballistics chart I was reading only had civilian ammo types!
3. I only selected the Match grade ammo types.
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