Height over Bore or How to Miss The Easy Rabbits

The Problem

This all started when I was working on the data for a previous article called “Optimal Barrel Length” where I found that a 8-10” barrel was optimal for a .22lr. At around the same time my youngest boy got his hunters education certificate and I found myself with two young hunters and only one kid sized rifle.

The Solution?

I am a fan of the black rifle (man legos) and thought that a pistol sized AR in .22LR might fit the bill nicely for a smaller hunter. With the availability of adjustable length pistol braces, an AR pistol makes a kid sized rifle that is a blast for dad to shoot as well!

To the Hunt

So I proceeded to build a 9” barreled AR Pistol and was happy with its accuracy at 25 and 50 yards and I felt confident that it would make a good small game gun. During the first rabbit hunt the first couple of bunnies went down as expected but there was an issue when it came to hitting the close ones (5-10 yards). As it turns out we were seeing the effects of the Height Over Bore.

To the Quantification

Height over Bore is the height of the sights over the axis of the bore and the AR platform has particularly high HOB.

In an attempt to quantify exactly what was occurring I went to a local range and took rested shots at 25y, 10y and 5y to see what the actual difference would be. I found that with a 25y zero I was hitting -1.5” at 10y and -2.25” at 5y and considering a 2.5” head on a standard Cottontail either one would be a miss without compensating. This is not a .22lr problem or an ammunition thing. I would expect the same results from a 223.

In the scheme of things hitting a deer or hog 2.25” low will still result in a clean kill, but in terms of Rabbits and Grouse it can mean an empty game bag.

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Standard HOB of a Oregon Arms Chipmunk

Standard HOB of a Oregon Arms Chipmunk

Standard AR HOB

Standard AR HOB

Same Point of Aim but very Different Point of Impact

Same Point of Aim but very Different Point of Impact

Not Much Room for error

Not Much Room for error