[IMT]

...Right now I make this part on the mill by holding the stock vertically in a three-jaw chuck, it's a few thou out of round this way but good enough for my purposes...

I'm in a similar situation, occasionally need a CNC lathe and use the "Mill as Lathe" feature. I've never had any accuracy issues though. Turned parts come out round within a few tenths. You got major spindle issues if it can't turn a round part.

[sansbury]

You got major spindle issues if it can't turn a round part.

I'm milling the parts with round stock held in a chuck as a vise.

[IMT]

I'm milling the parts with round stock held in a chuck as a vise.

That makes sense then. I misinterpreted what you were saying.