A-BOM (Adjustable BOre axis Mount)
The Bobro Engineering A-BOM is an adjustable system designed so that an electro-optical device can be centered over the bore axis of the host weapon. Once configured, the A-BOM is a semi-permanent interface, and is built to withstand both impact and pressure.
Most offset mounts do not account for the bore axis of the host weapon, and severely limit the capabilities of sight utilization. If the sight is not set on bore axis, the user is limited to a set zero distance, and any deviation from this point of aim/impact will introduce left and right corrections along with a simple elevation change. The A-BOM, once set, only introduces elevation changes to account for, and follows the function of the primary optic. For example, the A-BOM can be zeroed for 10 meters, but be utilized accurately at hundreds of meters. Only intuitive elevation changes are made.
The A-BOM uses the proven Bobro Engineering “counter rotating fastener” method to secure the center link of the mount. By removing the center link, the A-BOM can be configured in a “straight up” position and can be used in a conventional manner.
First, clear the weapon and remove the bolt carrier group or render it inoperable in some manner. We've found that an easy way to get the sight almost perfectly over bore axis is to stick a 3/16 L-Key in the barrel (perfect diameter for 5.56) and align the protruding part of the wrench with the red dot. Anywhere in the 360 degree rotation of the wrench will be on bore axis and if the sight corresponds to this same angle you'll be close to spot on. Fine tune adjustments can be made if necessary, but shots out to distance were made with this installation procedure with no fine tuning required. Torque the counter rotating fasteners to 45 inch pounds. Once tested and zeroed, remove one screw at a time, paint with high strength loctite, and re-torque. Repeat for all 4 screws.