Turning down the muzzle end of the barrel.
I looked this up in Machinery's Handbook #24
M14X1.5 (M14X1 is not listed) external metric thread M profile limiting dimensions=
Major Diameter Minor Diameter
Max Minimum Max Minimum
13.968mm 13.732mm 12.344mm 11.930mm
Metric conversion: mm to inches
1mm = 0.03937008"
So it would appear that the maximum outside diameter of your barrel should be:
13.968mm times 0.03937008" equals 0.5499201"
You said that your barrel measures 0.610", so:
0.610" minus 0.5499201" equals 0.0600799" (or about sixty thousands of one inch.) That is roughly the thickness of twenty sheets of paper stacked one on top of each other.
Brownell's sells a Barrel set back fixture that is ment to cut barrel shoulders without the use of a lathe. (See part number 080-762-380,Page 121, catalog number 55, $111.11.) Perhaps a simular fixture could be made that would turn the circumference of the barrel. Like a Foster neck turning tool. (Used to turn the outside of brass cartridges in the neck area.)
I don't really believe that you really want to try to remove all of this material by hand. You really should get a machinist to turn it down on a lathe.
That is just my two cents.
Thumb Clip Pull Pin.