The ops reamer has a screw at the out end so not going to rest on the tail center if running the bushing on the reamer.
Putting the reamer shank end in a collet likely will not make the far out end run true if some how not held.
Looks like the reamer is already made and the Op is trying to make the brass bushing to go on the end...and make it to run true.
If there is a center under that screw then the reamer holding the bushing might be run between centers.
looks like he is getting .0015 to .003 error now. I don't think he said how he is doing it.
*But I do agree if the bushing hole is made dead center and he skims the OD in one holding of the part and then parts it off it will be good/true and straight. Ye, this just doing the bushing alone.. That is why I said he might tool-bit scalp the drill hole ..or bore the hole so it will be center..Agree held in any chuck or collet.
He is actually trying to make
the pilot bushings, and you are trying to make it very much more complicated than it has to be. A LOT more.
Maybe you have seen replaceable non-rotating pilots on reamers, but I can say, I never have. Any I have seen with a screw in the end, were MEANT to rotate.
The very LAST thing he needs, is to be dicking around with trying to move the bushing from the parent stock at any time before he is ready to part it off.
Nor is he going to 'run the end of the reamer on a center'. It's a pilot
for a chambering reamer, so it does actually need to be reasonably concentric, and with the runout he claims, and without any idea how he managed to get it so bad, the best option I see, is to do all the Ops in one set, then part off the far end. That end can go towards the screw, where it won't even sit crooked on the reamer, if it somehow gets buggered up.
He posted pictures of the pilot bushing he intends to replace.
It doesn't matter what his intended clearance is, he wasn't asking that. He wants to know how to make the inside and the outside, concentric.
My best guess is that he did what pretty much every guy new to lathe stuff does, stuck a drill into the stock, then reamed the hole, and called it good and moved on through the process. That works great for making steel spacers for a welding project, but not so much for when the concentricity counts.