Scope ring height
The objective bell has to clear the barrel, and the scope has to sit as low as it can while it does. Enter four measurements and see the stack drawn to scale, with the clearance gap measured.
Measurements
Ring height here means rail top to the bottom of the tube. Manufacturers are inconsistent — some quote to the tube centre instead, which reads about half a tube diameter taller. Check which convention your rings use before ordering.
Fit
The geometry, and where it stops
Minimum ring height is (bell OD − tube OD) / 2 + clearance. Everything else on the diagram follows from that: tube centre sits half a tube diameter above the ring seat, and height over bore is rail height plus receiver-to-bore plus tube centre.
Height over bore is the number that matters afterwards. It is what you enter as sight height in a ballistic solver, and it sets how far below the line of sight your bore points at close range.
Not modelled
- Barrel contour. A heavy palma barrel eats clearance a sporter does not.
- Bolt handle and throw clearance under the eyepiece.
- Cheek weld. The lowest mount that clears is not automatically the one that lets you get behind the rifle repeatably — an adjustable comb changes the answer.
- Canted bases. A 20 MOA rail lowers the objective at the front slightly.
Measure your own bell rather than trusting a spec sheet. Advertised objective sizes are the lens, not the housing, and the housing is what collides with the barrel.