Return to Zero (RTZ)
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Return to Zero – often abbreviated RTZ – describes a mount's ability to provide the same point of impact before and after it has been removed. The term is especially used for quick-release mounts, but it is not a standard feature. RTZ requires several factors to align.
What is the basic principle?
When a scope mount is removed and reinstalled, the mechanism must be precise enough for the scope's optical axis to return to the same starting point relative to the barrel. If the mount is misaligned by even a fraction of a millimeter, it translates into a noticeable impact shift at long distances.
What does RTZ require?
- A standardized rail: The slot width and slot spacing must be within MIL-STD-1913 tolerances. The standard is described in MIL-STD-1913 vs. STANAG 4694, and the dimensions are found in Picatinny rail measurements and dimensions.
- A mount with a tight fit: The cross-pin must fit with minimal play in the slot, and the clamping mechanism must consistently pull the mount against one side of the slot.
- The same position on the rail: The mount must be placed on the same slots when removed and reinstalled. Changing slots breaks RTZ.
- Correct torque: A quick-release that doesn't lock securely will shift under recoil. An overtightened quick-release will wear down the slots and degrade the fit over time.
Picatinny vs. Weaver and RTZ
RTZ works reliably on Picatinny rails with a Picatinny mount because the slot width and slot spacing are consistent. Weaver rails have varying slot spacing, which is one of the technical reasons why RTZ cannot be expected with mixed use. The difference is explained in Picatinny vs. Weaver and specifically for mount selection in scope mount: Picatinny vs. Weaver.
How to test RTZ
- Zero your rifle at your typical distance and record the group.
- Remove the scope and mount.
- Reinstall the scope and mount on the same slots.
- Shoot a new group without adjusting the turrets.
- Repeat steps 3-5 times and note the shift.
A good QR mount on a Picatinny rail typically results in a shift of less than approximately 1 MOA. A larger shift indicates either a worn cross-pin, deformed slots, or a mount not designed with RTZ in mind.
How much difference does it make in the field?
| Shift | 100 m | 200 m | 300 m |
|---|---|---|---|
| 0.5 MOA | approx. 14 mm | approx. 29 mm | approx. 44 mm |
| 1 MOA | approx. 29 mm | approx. 58 mm | approx. 87 mm |
| 2 MOA | approx. 58 mm | approx. 116 mm | approx. 174 mm |
An explanation of the unit is found in MOA vs. MIL vs. degrees.
RTZ with night vision optics
RTZ is particularly important if you switch between a day scope and night vision optics on the same rifle. Each optic should have its own ring mount or adapter, and each should always be placed on the same slots. It is customary to adjust the night vision optic's own aim-point, not the scope's turrets, when switching.
What can compromise RTZ?
- Changing slots during removal and reinstallation.
- Worn or deformed cross-pin.
- Worn slots in aluminum rails with hard anodizing that have been used with a hard steel cross-pin.
- Loose rail screws between the rail and receiver.
- Damage to the clamping mechanism of the mount itself.
RTZ and mount selection
If RTZ is critical, it argues for a stiffer mount, preferably a one-piece steel mount with QPQ. A full comparison is found in steel vs. aluminum. The choice between fixed and quick-release is discussed in fixed mount vs. QR/QD.
Rail condition
The rail is an active part of RTZ. Keeping the rail's surface and slots clean helps maintain the fit longer. The complete routine is described in Picatinny rail maintenance. Always follow the manufacturer's mounting instructions for torque. As a reference, many 6-48 base screws are around 15-20 in-lbs, while 8-40 screws are often higher. Do not apply Nm values indiscriminately to small base screws.
What RTZ cannot do
RTZ is not a substitute for proper zeroing. It does not compensate for a change in ammunition types, a scope that has been adjusted to "empty turrets," or a rifle that has been internally adjusted. It only restores the physical position of the scope relative to the barrel.
Use in long-range shooting
For long range, RTZ is especially important if you move the scope for training and then back to the same rifle. The rail's cant can provide more usable elevation range, but RTZ still depends on the mount returning to the same mechanical position.
Quick checklist
- Are you using a Picatinny rail with a Picatinny mount?
- Is the mount seated on the same slots every time?
- Are the cross-pin and clamping mechanism in good order?
- Have you tested RTZ over 3-5 cycles?
- Do you keep the rail clean and lightly lubricated?
What is a good RTZ value?
There is no official standard for what constitutes a good RTZ value. In practice, the following unwritten guidelines are used:
| Shift over 3-5 cycles | Assessment |
|---|---|
| Under 0.5 MOA | Very good |
| 0.5-1 MOA | Satisfactory |
| 1-2 MOA | Acceptable for classic hunting, not for precision use |
| Over 2 MOA | Not acceptable – investigate cause |
How many cycles should you test?
A single removal and reinstallation cycle doesn't tell much. Three cycles reveal a pattern. Five cycles provide a reliable assessment. Each cycle should be a full removal with a short pause in between – not just a brief lift.
Slots and slot shape
RTZ depends on the mount seating in the same way every time. This requires the slot shapes to be consistent along the entire length of the rail and that the manufacturer has adhered to tolerances. If some slots are wider than others, the mount will seat differently depending on where it is placed.
Poor RTZ is therefore often due to inconsistent slots and not solely the choice of material.
The cross-pin – the critical element
The cross-pin locks the mount longitudinally. If the cross-pin is undersized or worn, the mount can shift in the slot, even if the clamping mechanism is locked. This is especially important for QR mounts, where the cross-pin is often a hardened steel pin that can, over time, wear the slot itself in a softer aluminum rail.
When a mount does not perform RTZ
If tests show an excessive shift, you should check in this order:
- Is the mount seated on precisely the same slots? A single slot forward or backward breaks RTZ.
- Is the cross-pin clean and free of oil?
- Is the mechanism tightened to the same torque every time?
- Are the rail screws still secure?
- Is there visible deformation on the slots or cross-pin?
Influence of ammunition and scope
RTZ only measures the mount's physical ability to return to the same position. It does not measure the scope's own reliability. If the scope's turrets have internal play, or if the ammunition varies in pressure, it can appear as "lack of RTZ" – but it's actually a different cause. It's worth separating:
- Change the setup and check if the problem shifts.
- Shoot with consistent, standard ammunition over several series to isolate the variation.
- Check the scope's turrets for their own RTZ promise – not all scopes have it.
RTZ and MOA cant
Cant does not change RTZ. The slots and the mount's fit remain the decisive factors. Cant changes the scope's working range, but not the mount's ability to return to the same mechanical position.