Kees, I am truly humbled.
WOW!
Fire Lapping provides, obturation, or "Bump-Up" to the ball...once you have bump-up, you automatically get "engraving" on the ball, which in turn means that a small part of the patch with the abrasive on it, is actually riding inside the rifling, and not just riding on the bore, which is what happens when you hand-lap.
The problem with this, if such can be said, is the fact that
if it's excessive, it can possibly effect the depth of the rifling.
By hand-lapping, you actually smooth the bore itself, which adds greatly to accuracy. because the rifling itself has already been cleaned by a recent fire-lapping...however, even if you didn't fire lap before hand, if that patch is too tight, or if you should have the desire to go 3-400 strokes, that barrel can also show excessive signs of wear....modesty is the name of the game in this process, because a 'little dab' goes a long way, even though you can't see it with the naked eye.
Fire lapping is often frowned on by many because they are afraid of somehow damaging the bore.
I can certainly imagine that too course, or too much, of an abrasive could very well do some damage.
However, the VGC that I have always used has a grit of 220, the "purple" tube...the "red" tube has a grit of 110 and I'm not real sure I would want to go there, as that grit is much more aggressive.
Due to the abrasive action of VGC, it is "typically" limited to 8-10 shots, sometimes even less, when fire-lapping, and I would never recommend more than this without some really good measurements prior to doing so.
The very best lapping can be done with a lead slug from the same barrel using Rottenstone for your abrasive...but that is not something the 'average' shooter has a way, or even a means to do.
Don't let this scare you off........
If your barrel once shot real well, or if it never shot very well at all, right out of the box, a proper hand lapping could result in "being just what the doctor ordered" for that extra accuracy.
Uncle Russ...