It’s probably not the scope

"It won't hold zero" describes a symptom, not a cause. The rifle, the mount, the optic, the ammunition and the shooter all sit in the chain between intention and impact, and any of them can produce the same symptom. Here they are in rough order of how often they turn out to be the culprit.

1. Ring cap and base screw torque

By a wide margin the most common cause. Screws that were snugged by feel rather than by an inch-pound driver either back off under recoil or were over-torqued into deformation from the start.

Pull the optic, degrease every mating surface, and re-torque to the specific values your hardware ships with — not a number from an article. Base screws and ring cap screws have different specs and are frequently an order of magnitude apart in the ranges you'll see. Tighten cap screws in a cross pattern, in stages, keeping the gap even on both sides.

Mark the screws with a paint pen afterwards. Then a glance tells you whether anything has moved.

2. Action screws

Chronically overlooked. If the action is moving in the stock or chassis, the barrelled action and the scope are no longer in a fixed relationship, and no amount of optic tightening fixes it.

Check the manufacturer's action screw torque and apply it with a proper driver. Bedding condition matters here too — a stock that has swelled, compressed or cracked around the recoil lug will let the action shift no matter what the screws read.

3. Barrel contact with the forend

A free-floated barrel should be free-floating. Slide a dollar bill or a business card along the full length of the barrel channel from the forend tip back to the recoil lug. Any contact point changes barrel harmonics as the forend flexes under a bipod, a sling, or a rested hand — and your group centre moves with your hold.

This one presents as "the zero changes depending on how I shoot it," which people naturally attribute to the scope.

4. Parallax

Not a zero-retention problem at all, but it produces the same complaint and it belongs here because it's so often the answer.

Parallax is the apparent movement of the reticle against the target when your head shifts behind the scope. If your parallax is set for 300 yards and you're shooting at 100, moving your cheek a quarter inch moves your point of aim measurably. The impacts scatter, the shooter concludes the scope is unreliable, and nothing is actually wrong with it.

Set parallax at the actual distance you're shooting. The correct method is to adjust until the reticle stops floating against the target when you deliberately move your head — not until the image looks sharpest, which is a different setting and a common mistake.

5. Reticle cant

A reticle that isn't level with the rifle's vertical axis introduces horizontal error that grows with every click of elevation. At 100 yards it's invisible. Dialled up for 800, a couple of degrees of cant walks you off the target laterally, and it will look like the windage isn't holding.

Level the action, then level the turret cap to it. Do this properly once rather than eyeballing it against a doorframe.

6. Mount and rail mismatch

• Picatinny rings on a Weaver rail, or the reverse. The slot dimensions differ. A Picatinny recoil lug in a Weaver slot has room to move under recoil.
• Rail not properly bedded to the receiver. A one-piece base bridging a receiver that isn't flat will flex.
• Ring height forcing a compromised cheek weld. Not a mechanical fault, but it produces inconsistent head position, which produces inconsistent impacts.
• Aluminium rings over-torqued onto a thin scope tube. Crushing the tube can bind the erector assembly and cause genuinely erratic tracking.

7. Ammunition

Different lots of the same factory load can shift point of impact measurably. Different bullet weights will shift it a lot. If your zero "drifted" and you also opened a new case of ammunition, that's your first suspect, not your last.

Temperature-sensitive propellants shift velocity between a cold morning and a hot afternoon, which shifts elevation at distance. This is normal physics, not a fault.

8. The shooter

Uncomfortable, but it belongs on the list. Inconsistent cheek pressure, inconsistent bipod loading, a changed rear bag position, or shooting off a different rest than you zeroed from will all move the group. So will fatigue.

The way to separate shooter from equipment is to have someone else shoot the rifle, or to shoot it from a genuinely mechanical rest. If the group tightens and stays put, the equipment is fine.

Only now, the scope

If the mount is properly torqued, the action is solid, the barrel floats, parallax is set correctly, the reticle is level and a second shooter reproduces the problem — run a box test. If the scope doesn't return to zero after dialling out and back, the erector assembly is at fault and the optic needs service. Most reputable manufacturers cover this.

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