A civilian is held to a lower standard of weapons knowledge than a police officer.
A person (the BG) displaying any item that is 'reasonably believed' to be capable of inflicting deadly force, held in such a way that could inflict deadly force (ie. pointing it) is considered to be commiting an act of deadly force.
In that instance, the target of this deadly force (the GG) can use deadly force to stop the illegal activity.
If you use deadly force, the resulting investigation may actually include items that attest to your expertise (or lack thereof) with firearms. Including books, magazines, training certificates, military experience (including your MOS), etc.
Police officers are assumed to have a certain level of expertise.The same with former combat soldiers (as opposed to medics, engineers, supply clerks).
Civilians (including legal gun owners) are not assumed to have expertise unless the above items (boods, magazines, etc) are present.
It's all what the jury can be convinced of.