Part of my moldmaking career included being sent to Chicago to understudy a master mold and die finisher.He was working on a coining die for the mint when I showed up.
Regarding the OP's scratch.A surface finish has high spots,low spots,and uniformity.A chamber reamer leaves a tool finish with the lines running perpendicular(pretty much) with the bore axis.To be able to see a flaw in the PATTERN,the lines,may indicate the tops of the texture have been disturbed,This can be visible with practically no dimensional disturbance.If the flaw is very narrow,less than the case wall thickness,it is unlikely the case will flow into it at reasonable pressures.Think in terms of a 1903 Springfield with a cone breech or a handgun with unsupported chamber.
At least now your chamber is round,form and dimension correctness.Polish the low spot out of your scratch,you will likely compromise form and dimension. When I polish,I carefully polish off the high spots till all the low spots disappear at the same time.Then the low spots define form and dimension.
Now,as your tool marks ordinarily are perpendicular to the bore for direction,if you use a mop or soft media with abrasive,as it cuts the tops off the tool finish,it also conforms to the low spots on the tool finish and cuts them deeper!
You must stroke in the direction of the bore across the lines of the tool finish.
You want the stone or lap to bridge over the low spots ,and not cut them.You only want to cut the high spots
The first,essential step to a good polish is a good tool finish.Cut abad chamber with a dull reamer,or straddle a chip over a cutting edge,you aren't going to fix it polishing.
I ordinarily do not polish a chamber,but,if I did,I would run the lathe as slow as it will go in back gear,dress a 600 or 800 grit soft mold polishing stone ,use kerosene,and uniformly stroke the length of the chamber as it rotates.
This will make a crosshatch.Inspect frequently,only cut 15 seconds or so at a time.As soon as you have a uniform change from cutter marks to crosshatch,stop.
I would do nothing more,other than the radius corner beak at the chamber mouth.
Last edited by HiBC; January 8, 2013 at 06:34 PM.