I do not know what a barrel crown is nor what it does. I do know that if you cut threads off a preban it needs crowned. Please explain this to me so I can further understand barrels.

The crown is a bevel cut surface on the bore at the muzzle.  I'm not sure what angle they are cut at, and if it differs for different calibers, but its purpose is to evenly distribute the exiting gasses against the base of the bullet as it leaves the barrle. If the crown is chipped, the gas escaping from the muzzle will not be evenly dissipated 360 degrees as it exits the bore.  This would apply more force to one side of the base of the slug than the other.  Obviously,  accuracy goes down the tubes under such circumstances.

I get it. I guess it's kinda commen sense. This AR crap is just a hobby of mine and I keep learning more and more. Thanks for the light.I shall now seek more light.
         Chitty