MagnetoSpeed's barrel-mounted ballistic chronographs use patented electromagnetic sensors that are designed to operate in the harsh environment at the muzzle of firearms. Our chronographs shrug off common environmental conditions that bedevil other products, such as too much/little sun, fluorescent lights, nearby shooters' muzzle blasts, high winds or many other unpredictable variables. Using the provided spacers, the shooter adjusts the bayonet so that the bullet will travel just above unit’s sensors. Each sensor produces a voltage that is proportional to the change in its magnetic field. The display unit is able to use these voltages to sense the approach of the bullet and accurately determine the time the bullet was directly over each sensor. Because the distance between the two sensors is precisely calibrated,+/-.005 of an inch, the time of flight between the sensors gives a very accurate velocity of the projectile.