Return to Zero (RTZ)
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Return to Zero – often abbreviated as 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 an inherent property. RTZ requires several factors to align.
What is the basic principle?
When a scope mount is removed and reattached, 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 point of impact shift at long ranges.
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 detailed in Picatinny rail dimensions.
- A tightly fitting mount: The cross bolt must fit snugly in the slot with minimal play, and the clamping mechanism must consistently pull the mount against one side of the slot.
- Same placement on the rail: The mount must be placed on the same slots when removing and reattaching. 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.
- Reattach to the same slots.
- Shoot a new group without adjusting the turrets.
- Repeat 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 signals either a worn cross bolt, 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 optics
RTZ is especially important if you switch between a day scope and night 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 common to adjust the night optic's own aim-point, not the scope's turrets, when switching.
What can compromise RTZ?
- Changing slots when removing and reattaching.
- Worn or deformed cross bolt.
- Worn slots — regardless of material — can affect RTZ. This is primarily relevant with extremely frequent use over many years.
- Loose base screws between the rail and receiver.
- Damage to the clamping mechanism in the mount itself.
RTZ and mount selection
If RTZ is critical, a rigid mount, preferably one-piece, is recommended. Both steel with QPQ and aluminum with hard anodizing provide reliable RTZ under normal use. A full comparison is available in Steel vs. Aluminum Picatinny. The choice between fixed and quick-release is discussed in Fixed mount vs. QR/QD.
Condition of the rail
The rail is an active part of RTZ. Keeping the rail's surface and slots clean prolongs the fit. The full routine is detailed in Picatinny rail maintenance. Always follow the manufacturer's mounting instructions for torque. As a reference, many 6-48 base screws are torqued around 15-20 in-lbs, while 8-40 screws are often higher. Do not indiscriminately use Nm values for small base screws.
What RTZ cannot do
RTZ is not a substitute for proper zeroing. It does not compensate for switching ammunition types, a scope that has returned to "empty turrets," or a rifle that has been internally adjusted. It only re-establishes the physical placement of the scope relative to the barrel.
Use in long-range shooting
For long-range shooting, RTZ is especially important if you move the scope for training and then back to the same rifle. Rail inclination 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 placed on the same slots every time?
- Are the cross bolt and clamping mechanism in good order?
- Have you tested RTZ over 3-5 cycles?
- Do you keep the rail clean and lightly lubricated?
If RTZ is to be tested seriously, the rail, mount, and screws should first be correctly installed and checked for play.
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 |
|---|---|
| Below 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 reattachment cycle doesn't tell much. Three cycles show a pattern. Five cycles provide a reliable assessment. Each cycle should involve a full removal with a small 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 presupposes that the slot shape is uniform 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 bolt – the critical element
The cross bolt locks the mount longitudinally. If the cross bolt is undersized or worn, the mount can shift in the slot, even if the clamping mechanism is locked. With QR mounts featuring a hardened steel cross bolt, the slots may show signs of wear over time with very frequent use. In practice, this is rarely an issue under normal hunting conditions.
When a mount does not perform RTZ
If testing shows an excessive shift, you should check in this order:
- Is the mount placed on the exact same slots? A single slot forward or backward breaks RTZ.
- Is the cross bolt clean and free of oil?
- Is the mechanism tightened to the same torque every time?
- Are the base screws still tight?
- Is there visible deformation on the slots or cross bolt?
Influence from 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 ammunition pressure varies, it may appear as "missing RTZ" – but it is actually a different cause. It is worth distinguishing:
- Change components and check if the problem shifts.
- Shoot with consistent, standard ammunition over several series to isolate 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.