One of my favorite RPG source books was the "Compendium of Modern Firearms" written by one of the Morrow Project Authors Kevin Dockery.
It was particularly valuable for anything involving snipers as in addition to the normal stats for a weapon (weight, num of rounds, ammo type etc) it also presented the Dispersion Angle of the weapon.
No weapon fires perfectly straight. Sniper's rifles are much better than most but even a variation of 0.03 degrees will lead to a very large circle of potential deviation at 1200 meters range. This will mean luck is in the equation when you are dealing with extreme range limits of a weapon.
This graphic shows the effects of that Deviation when using a M-21 with match grade ammo. You can see that even when a rifle is bolted down and not effected by wind conditions there would only be a ~72% chance of hitting a body sized target at 1200 meters. When you add other variables like wind, human instability, and the over 40 foot drop the bullet would take over that distance the numbers seem right to me.
Other rifles are much better than the M21 in terms of deviation so those might be a better choice for a really long range shot.