What is the most common size of a sight base screw?

The most common size of a sight base screw is 6-48. This designation specifies the screw's diameter (6) and the number of threads per inch (48). The 6-48 size is frequently used in various firearms and optics applications, as it provides a good balance between strength and ease of installation.

It is important to understand that other screw sizes, while they may be used in specific applications, are not as universally adopted for sight bases. The 8-40 and 10-32 screws, for instance, are larger and not as prevalent in standard sight attachments. The 4-36 screw is smaller and typically not strong enough for the demands placed on sight bases, making it a less common choice for this purpose. Thus, 6-48 remains the most reliable and widely used option for securing sights to firearms effectively.