The EBC... What is it?
The EBC uses electromagnetics to measure variations in jacket thickness and outer jacket roundness of a projectile.
How does it do it?
When you put a metal object such as the copper jacket of a projectile into a magnetic field there are eddy currents generated in the projectiles jacket. The strength of that eddy current is is dependant upon the amount of metal and how close to the source of the magnetic field it is placed.
The eddy currents that form in the projectiles copper jacket push back on the magnetic field. They cause an impedance that we can detect.
As the EBC rotates the projectile in the magnetic field it measures the variations in how much that impedance impacts the magnetic field. That is translated into a number that we can use to grade our projectiles.
Other metals that have electromagnetic properties are brass, steel and aluminum. Lead, which forms the projectiles core is not affected by magnetism and is effectively invisible to an eddy current sensor like the one the EBC uses.
Electromagnetically speaking, copper, brass, steel and aluminum all behave differently to each other. The EBC can only handle copper projectiles and cannot be used precisely for other materials. That may seem like a limitation but it is infact a benefit.
Being tuned specifically to copper jacketed projectiles allows the EBC to measure jacket thickness more precisely. The alternative would result in the EBC placing significntly more emphasis on measuring the external roundness variations of a projectile rather than jacket thickness variations.
Why does it matter?
Projectiles consist of a thin copper jacket with a lead core. If the jacket is perfectly centered within the jacket the projectile will spin consistently in flight and will have a Ballistic Coefficient (BC) closer to it's ideal value.
If the lead core is off-centre within the projectile then the projectile will take longer to stabilise after leaving the muzzle and will continue to have a wobble. The consequence of which is a lower Ballistic Coefficient and the impact that that will have on the flight of a projectile.
How accurate is it?
That's a bit of a tricky question to answer...
The problem is twofold. The first is that it can measure more precisely that my most accurate mircometer which measures to 0.0001". The second issue is that any reading is impacted by a value that cannot be observed which is the thickness of the jacket material. So it's impossible to say that a particular value equates to a specific jacket thickness.
So it's pretty accurate but it's impossible to quantify. This means that it is very good as a comparative device but not for absolute measurements.