Rich,
I made a receiver insert today, and after some measuring and test fitting I found that .540" x .540 was a snug fit. I figured that the firing pin is most likely centered, so I drilled a hole and chucked it up in the lathe (barrel band in the 3-jaw chuck and with a live center in the insert). With an indicator at the rear end of the taper (right before it rounds over), I read a .006" TIR side to side. Top to bottom was only .002" TIR, which is not too bad considering that the barrel has some slight pitting. I turned the insert 180 degrees and tried again, and got the same result. The low reading was on the same side regardless of how I turned the insert, so it seems like the bore for the bolt is off center. The point I measured is about 1/3 from the muzzle, which means that the rear of the receiver is off by .009".
Have you tried to measure the runout on an intact barrel/receiver, or do you know what a normal runout would be?
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