320 is considered industry standard. 400 is extra fine and considered by many as so fine it's hard for the brass to grip and will cause excessive bolt thrust. I call BS. The excess bolt thrust thing is a myth. The brass is only a bladder like an inner tube. Having the chamber super smooth hurts nothing. Polish it like chrome if you want. It hurts nothing. I think 400 is perfect. Keep in mind that with 400 the brass will leave marks in the chamber. It will look all kinds of scratched up even with pristine brass. After a few hundred rounds the marks will start to blend together and not look so bad.
Under normal pressure brass springs back .001" so it should never get really stuck. Excessive pressure will stretch things out beyond what the brass will spring back and that when they get really stuck. Ripping the rim off stuck. This is why I really dislike cutting the shank section out of a barrel for the purpose of saving weight. A high pressure round needs that bulk over the chamber to contain the pressure. Another factor in sticky bolt opening can be if the lower ramp has been cut on to aid feeding long rounds. That will allow that lug abutment to flex and that will allow the brass to expand lengthwise causing a sticky bolt opening.
The excess bolt thrust thing is a myth. The brass is only a bladder like an inner tube. Having the chamber super smooth hurts nothing.
You and I are on the same page of music,....
I've almost always started with 320,...and than gone progressively finer, as the spirit moved me.
If one doesn't understand the term uniform crosshatch, and surface speeds as it directly relates to the process of lapping, the wise thing to do is bring your recalcitrant chamber to someone who does.
I've got 3M Emory paper all the way out to 2000 grit,...there ARE times and places places where that comes in handy.
GTC
