YouTube / The Daily Aviation
The US Army uses bore sighting devices to calibrate a tank’s main gun with the fire control system.
Tanks like the M1 Abrams, which has a 120mm gun, are boresighted using the M27A2 MBD. Meanwhile, tanks with 105mm guns use the M26A1. The only difference between the two is the expanding adapter collet – which is more prominent in the M27A1 to accommodate the bigger barrel.
A crewmember first fits the device to the barrel, looks into the eyepiece where there’s a 45-degree mirror and a crosshair, and directs the gunner where to aim until the crosshair is on target. Once that’s done, the gunner inside the tank adjusts his sights, without moving the gun, so that it is also aiming at the same target.
Boresighting is usually followed up by a live fire exercise to confirm the tank’s accuracy.