The crown has 2 functions.
1. To protect the rifling
2. So that gases/ air escapes evenly.
A crown isn't exactly necessary. I have cut down barrels in small increments testing velocity and only deburred the fresh edge of the rifling as to not effect velocity and was surprised how accurate it still shot. Keep in mind I was not removing the barrel, so I used either a hacksaw or a cut off wheel, so the cut was not perfectly straight either.
A damaged crown can cause inaccuracy. However, similar to my above example, I have seen crowns that looked terrible yet still shot very well. Though a a smooth even crown is always the goal.
I do believe a proper crown is important. A damaged crown can be fixed in different ways depending on the damage. A well cut crown that has a slight imperfection can be polished or lapped to remove the bad area. For this without removing the barrel, I use a polishing cone with a polishing compound.
I prefer to Chuck a barrel in a lathe and use a brass lap tool with compound for touch up work.
If the crown is really bad or it is a new barrel blank, I will square up the muzzle face in the lathe then cut a new crown which I finish by lapping, then polishing. A purpose made crown cutting tools can be used or a homemade lathe tool.
