Tube Diameter
Also: 30mm · 34mm · Main Tube
The diameter of a riflescope's main body; larger tubes generally allow more internal adjustment travel and require matching rings to mount correctly.
Tube diameter refers to the outside measurement of the main body of a scope, the section between the bell and the eyepiece where the rings clamp. The common sizes are one inch, 30mm, 34mm, and the larger 35mm and 36mm bodies found on some long-range optics. A wider tube gives the internal erector assembly more room to swing, which is why high-travel scopes built for distance shooting tend to use the larger diameters.
The diameter you choose dictates the hardware around it, since scope rings are cut for one specific size and will not safely hold a tube of another. More internal room generally translates to more elevation adjustment on tap, though glass quality and erector design matter just as much as raw width. When planning a build, the tube size, the rings, and the scope base all need to agree before the optic ever reaches the rifle.