Bullets fly in arches, they are pulled down by gravity. If you would fire a bullet horizontally over a flat surface and at the same time drop now from the same elevation it will hit the ground at the same time.

the result is the scope does not look parallel to the barrel, the barrel will point slightly upwards if you aim horizontally. It looks like https://qph.cf2.quoracdn.net/main-qimg-20f8f9b2aa7051df653d04e40e2233c2.webp The image uses rear and from irons sight, a scope will create a line of sight the same direction as an iron sight

You zero a sight so the bullet trajectory and the line of sight are at a specific distance. Sight for rifles usually have an adjustable rate setting that is calibrated for the rifles and ammunition so the line of sight is changing relative to the barrel the correct amount so they intersect at the new distance.

So if you zero the rifle with so, for example, the 100-meter setting hits the line of sight at 100 meters when you then change the setting to 200m the bullet should hit the line of sight too. If the range is incorrect the bullet will hit above or below where you are.

If you can hold a weapon vertically all the time the distance the optics are above the barrel does not matter for accuracy. If you need to turn the weapons sideways it is better if you have them closer to the barrel but that is something that is more likely you need to do with assault rifles than sniper rifles.

The main problem with a large offset is to avoid hitting something just in front of the barrel. If you are on the uneven ground something could be in front of the barrel but not in front of the sight. Lay down on the uneven ground and try to expose your body as little as possible to enemy fire then you risk just seeing over some dirt that is directly in front of the muzzle. This is the