No first hand experience as my nitrided barrels were done when new. Some say it is better to shoot them in some in case there are any burrs or irregularities in the barrel. The negative is it is a lot of work to get a barrel 99+% clean if it has been shot. I have not done this but others recommend fire lapping before nitriding, this might make sense on a factory barrel or an older rough barrel.
Another dodge I have done is to scrub the throat with 0000 steel wool and JB bore compound on a cleaning rod in an electric drill. The idea is that any tool marks are likely to be in the throat and the rest of the barrel especially if hand lapped should be left alone.
The only down side to doing a used barrel is if you don't get it clean enough first. It will not improve a shot out barrel much if at all.
Hopefully some one who has done more barrels will respond.
