Measuring Height Over Bore

How to Measure Height Over Bore (HOB)

Height Over Bore (HOB) is the vertical distance between the centerline of the rifle bore and the centerline of the optic.

This measurement is important for ballistic accuracy, proper zeroing, and correct setup of ballistic optics.

Tools Needed
Digital calipers recommended
Ruler as a secondary option
Rifle with optic mounted

Step 1: Measure the Scope Diameter

Measure the outside diameter of the scope tube. Most scopes will use either a 1 inch, 30mm, or 34mm main tube.

Divide that measurement by 2 to determine the optic centerline radius.

Example:
34mm tube = 1.34 inches
1.34 ÷ 2 = 0.67 inches

Step 2: Measure From the Top of the Barrel to the Bottom of the Scope

With the optic mounted, measure from the top of the barrel to the bottom of the scope tube.

This gives a practical field measurement for the scope height above the barrel surface.

Step 3: Calculate Height Over Bore

Add the two measurements together:

Height Over Bore = (Barrel to Scope Bottom) + (Scope Radius)

Example Calculation:
Barrel to bottom of scope = 1.50 inches
Scope radius = 0.67 inches
Height Over Bore = 2.17 inches

Important Notes

• True HOB is measured from the center of the bore to the center of the optic.
• Measuring from the top of the barrel is an acceptable and consistent field method.
• Small errors can affect ballistic calculations and point of impact at shorter distances.
• Re-measure any time the optic, mount, rail, or rings are changed.

Typical Height Over Bore Ranges

Platform	Typical HOB
Bolt-Action Rifle	1.5" to 2.2"
AR Platform	2.1" to 2.8"

Quick Reference

Step	Action
1	Measure scope tube outside diameter and divide by 2
2	Measure from the top of the barrel to the bottom of the scope
3	Add both measurements together
Result	Height Over Bore

For assistance with ballistic optic setup or verification of HOB measurements, please contact a local competent gunsmith or firearms specialist.