Solid and live pilots are both used on finishing reamers. The live pilot reamers cost more. The live pilot can be used to within half a thousandth of the bore diameter of the barrel blank, where a solid pilot normally is ground one or two thousandths narrower than nominal bore diameter. This is to ensure fairly universal fit and to ensure enough clearance for a lubricating film of oil or cutting fluid to surround the pilot and allow it to slip freely inside the lands during cutting.
That description makes it sound like the live pilot has a significant advantage, until you remember it needs its own additional clearance between the pilot and the reamer for rotation. That is a pair of precision ground surfaces, allowing them to keep an oil film more concentric, so it's got the potential to be a little bit more true running than a solid pilot. I'm just pointing out that its not as much better as you'd think at first consideration of the matter.
The main advantage to the live pilots is you can get them in multiple sizes, so you can select one that best matches the particular blank you are cutting. I have one in .30-06 with a set of pilots and with oil grooves for through-bore lubrication on my lathe. I've found that in finish reaming less expensive grade barrel blanks, the ability to select the right pilot gave me tighter bore axis alignment. However, if I have a hand-lapped barrel blank with the groove diameter within a couple ten thousandths of nominal, and a solid pilot that has the tighter 0.001" clearance (you can normally get a reamer maker to give you what you want if you order direct) the advantage of the live pilot over solid gets a little more fuzzy.
