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How to match a mount to your rail and tube

Every mounting question resolves in the same order: confirm the rail, then the tube, then the height, and only then buy.

Step one, identify the rail

Start by naming the rail on the receiver. Measure the slot width: 0.206 inch with even 0.394 inch spacing is Picatinny (MIL-STD-1913); 0.180 inch with irregular spacing is Weaver. An angled groove with no slots, measuring 11mm to 13mm across, is an airgun dovetail. A 20mm flat-top on a modern sporting rifle is a Picatinny-pattern RIS.

The rail decides which mount bases can even seat, so this is the first gate. A Weaver base may fit Picatinny, but a Picatinny base rarely fits Weaver, and neither fits a dovetail.

Step two, identify the tube

Measure the straight main tube of the scope. It will be 1 inch (25.4mm), 30mm, 34mm or 35mm. The ring bore has to match this to the millimetre; a 30mm ring will not hold a 1 inch tube without a proper insert. If the optic is a red dot rather than a tube scope, identify its footprint instead and choose a plate cut for it.

Step three, set the height

With rail and tube fixed, choose ring height so the objective bell clears the rail and any iron sights with a small gap, while keeping a comfortable, repeatable cheek weld. A bigger objective needs taller rings. On flat-top rifles that need the scope pushed forward for eye relief, a cantilever mount does that job in one piece.

Pick the lowest height that still clears the bell, then confirm your head sits naturally behind the eyepiece.

Bringing it together

Run the three checks in order and the shortlist narrows itself: the rail rules out incompatible bases, the tube size rules out wrong-bore rings, and the height and objective rule out anything that fouls or forces a poor head position. Buy only once all three match rather than guessing on any one.

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Sources: MIL-STD-1913 (Picatinny) · Vortex