For a half thousandth difference I'd probably just put the barrel on there. As you start to dig into the <0.0005" range there are a multitude more things that come into play not the least of which are the headstock bearings and the shop temperature when you dialed it in vs. when you actually cut it or when you dialed it in last time.
0.0005" = <s>50</s> 500 millionths of an inch
Cte|SS ~6.5 millionsth-in/in-F
Meaning that a 1.000000" diameter piece of SS that is at 70F turns into 1.0000013" at 72F
For a receiver that is 1.35000" across at 32F outside and you bring it into a 72F shop that's 1.35*.0000065*(72-32) = 1.350351 which is <s>35/50</s> 351/500 millionths you're already chasing.
That's JUST the temp change for concentricity, what about the headstock bearings that may or may not be capable of holding the other 0.00015"? What about angularity, what about any potential flexure put into the action by your holding method, etc etc.
My advice is that you're going to spend a lot of time chasing this unicorn and the next time you check it, you're going to have to chase it again and again.
ETA: I made a mistake above, 1/2 thou isn't 50 millionths, it's 500 millionths. 1.35*6.5e-6*40 is also 350 millionths, not 35. I was off by 1 decimal place