Thread pitch describes the diameter and thread count of the mounting hole on a firearm barrel or rail, measured in millimeters (mm). The specific value you need depends entirely on the hardware you intend to mount the red dot sight onto.
Which Thread Pitch Suits Your Setup?
| If this is you | The Thread Pitch that suits you | Why |
|---|---|---|
| You are mounting to a standard 9mm pistol barrel. | 13.5x1mm or 1/2×28 (converted to mm) | These are the most common standards for handgun barrels to ensure the optic sits flush. |
| You are using a dedicated competition pistol. | 13.5x1mm | This provides a precise fit for high-recoil stability on the muzzle. |
| You are mounting to a standard AR-15 style rail. | M18x1mm | This is the standard for many modern accessory mounts and muzzle devices. |
| You are using a specialized hunting rifle. | 15x1mm or 16x1mm | Larger bores often require wider thread diameters to accommodate the barrel’s diameter. |
| You are using a custom-built precision rifle. | Specific to manufacturer | Custom builds may use unique pitches that require a specific adapter to match the optic. |
Red Dot Sights covered by our own reviews
Prices across these 3 run from $127.99 to $588.99; each one has a full review here.
Trijicon RMR Type 2 3.25 MOA
$588.99 price checked August 2026
Read our full Trijicon RMR Type 2 review
If you plan to add a magnifier to your setup, see our roundup of the best red dot sight with magnifier options.
AT3 Tactical RD-50 Red Dot + 3X Magnifier
For those requiring a rugged build, explore the best durable red dot sight for heavy duty use.
$149.99 price checked August 2026
Read our full AT3 Tactical RD-50 Red Dot + 3X Magnifier review
How Do I Measure My Thread Pitch?
You must identify the exact thread pitch of your mounting surface before ordering a red dot sight to ensure the optic can be secured. The measurement is typically found on the barrel’s threads or the mounting rail’s underside.
Locating Manufacturer Markings
Manufacturers often stamp the thread pitch directly onto the barrel or the rail system. Look for a series of numbers and letters near the muzzle or the mounting point, such as “M13x1” or “1/2×28”. If the marking is present, it provides the definitive measurement for your purchase.
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Using a Thread Pitch Gauge
If no markings exist, you can use a thread pitch gauge to determine the size. This tool features a series of pins or blades that correspond to different mm measurements. You rotate the gauge against the threads; the correct pitch will allow the gauge to seat perfectly without wobbling or forcing.
Measuring External Diameter
You can also use a digital caliper to measure the outer diameter of the threads. Once you have the diameter in mm, you can cross-reference it with standard thread tables. For example, a 13.5mm diameter usually corresponds to a 13.5x1mm pitch in many optic applications.
What Happens if the Thread Pitch is Wrong?
A mismatch in thread pitch means the red dot sight will either refuse to screw onto the mount or will sit loosely and dangerously. Because thread pitch is a physical geometry, there is no “close enough” fit; the threads must align perfectly to distribute the weight and recoil of the optic.
Risks of a Pitch That Is Too Small
If the thread pitch on the optic is smaller than the requirement of the mount, the optic will not thread onto the hardware at all. You will find that the threads seize or “cross-thread” after only a few turns. This prevents the optic from being mounted and requires a different mounting solution or an adapter.
Risks of a Pitch That Is Too Large
If the thread pitch is too large, the optic may appear to thread on but will have significant “play” or wiggle. This creates a dangerous failure mode where the optic can shift during recoil, leading to a “mount walk” where the red dot moves off-target. This happens because the threads are not grabbing the internal threads of the mount securely.
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What to Do if You Fall Between Two Thread Pitches?
When a measurement falls between two standard thread pitches, you should always choose the larger diameter of the two options. This ensures that the mounting hardware has enough surface area to remain stable under recoil.
Using Thread Adapters
If your specific barrel uses a non-standard pitch that does not match common red dot sights, you can use a thread adapter. These are small pieces of hardware that screw into your barrel and provide a new, standard thread pitch for the optic. This allows you to use a standard red dot sight on a unique barrel without needing a custom-machined mount.
Verifying Clearance Before Ordering
Before ordering, confirm the total clearance of the mounting surface. If the barrel diameter is very large, ensure the thread pitch you select has a large enough outer diameter to physically clear the barrel’s body. If the thread is too narrow to fit around the barrel, the optic cannot be mounted regardless of the pitch match.
Checking Local Rules
Before ordering any mounting hardware or optics, confirm the current regulations for your specific location. Rules regarding specific components can vary by region, and checking these before purchase ensures your setup remains compliant with local requirements.