Parallax: it's not your scope mount
When you start pushing a rifle out past three or four hundred yards, you run into a very specific optical problem. You get behind the rifle, get perfectly comfortable on the stock, and put the crosshairs dead center on the steel target. But if you move your head just a fraction of an inch to the left or right, the crosshairs appear to physically float and shift off the center of the target, even though the rifle hasn't moved a millimeter. A lot of shooters think their scope mount is loose or the internal erector tube is broken. The hardware is fine. What you are experiencing is an optical illusion caused by parallax, and if you don't know how to adjust for it, you will miss small targets at long distances.
To understand parallax, you have to look at how a scope manages the light entering the tube. The reticle—your crosshairs—is etched onto a physical piece of glass mounted permanently inside the scope. When you look at a target downrange, the large objective lens at the front of the scope gathers the light and focuses it backward into an image inside the tube.
Parallax occurs when the focal plane of that target image does not align perfectly with the physical glass of the reticle. The easiest way to visualize this is to think about the speedometer in your car. If you are sitting in the driver's seat looking straight on, the needle might show exactly 60 miles per hour. But if you lean over to the passenger seat and look at the speedometer from an angle, the needle suddenly looks like it's pointing at 55. The needle didn't move, and the numbers didn't move. Your viewing angle simply changed relative to the physical gap between the needle and the gauge face.
The exact same thing happens inside your scope. If the target image is focused slightly in front of or behind the reticle glass, a physical gap exists between the two. When your eye is perfectly dead center behind the scope, everything looks aligned. But the second you shift your head off-center, that physical gap causes the crosshairs to shift relative to the target image.
To fix this, modern long-range scopes feature a parallax adjustment knob, usually located on the left side of the turrets. When you turn that knob, you are physically moving an internal lens forward or backward inside the scope tube. The goal is to move the target's image plane until it converges perfectly on the exact same plane as the reticle glass. Once the image and the reticle are on the exact same physical plane, the gap is gone. You can move your head all around the eyebox, and the crosshairs will stay locked dead center on the target.
A lot of shooters confuse the parallax knob with a simple focus ring. While adjusting the parallax will often make the target look sharper, its mechanical purpose is aligning the focal planes, not just clarifying the picture. You should always use the diopter dial on the eyepiece to focus the reticle to your specific eye first, and then use the side knob to remove the parallax shift at distance. When I'm setting up a rifle for precision work, I pull my high-magnification glass from https://www.brownells.com/optics/scopes/ to make sure it has a reliable, repeatable parallax adjustment. Don't let optical illusions ruin your groups. Dial the knob, check for reticle shift by moving your head, and lock in the focal plane before you pull the trigger.