Do you mind sharing a source of the hard felt stock?
I've done a fair amount of fire lapping, and I don't find the bore reamer marks are usually more than about a ten thousandth or two deep (a slug shows it has opened up maybe a quarter to a half a thousandth at most by the time all the abrasive marks are scratched longitudinal. I assume most chamber marks, unless there's a real gouge, are similar in RMS surface roughness. Given the +0.002" typical SAAMI chamber diameter specs, unless your chamber is at that maximum, taking off a tenth to a quarter thousandth shouldn't really take the chamber out of spec. But you do need to know what you are starting with and I've seen a few pretty fat AR chambers (5.56 NATO spec) that I would not want to enlarge any.
I agree that using the reamer correctly in the first place is the main key. I got one of Greg Tannel's through-bore pressure cutting fluid systems years ago, and if everything is set up right on the lathe and you use a reamer whose pilot has lube passages ground in, you wind up with one very smooth chamber finish from the git-go.
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