I can usually get to +/- half a thousandth with my Chineseium hole gauges, plenty close for most of what I do. I have been knowed to turn and polish a pin to size on the lathe when the measuring is more critical. Pound casts have been the easy way to get neck dimensions, make sure the case neck/shoulder is good and annealed and the hard lead is only up to the neck/shoulder junction so the neck expands to the full dimension.
Let's see..... .253" ballpark neck diameter (there's some taper involved with the chamber neck, another advantage of the pound cast because you can find out exactly what it is, so your hole gauge measurement isn't exact). Brass is what....011" per side? Throat entrance somewhere around .225" unless cut with a 5.56 reamer, so that adds up to .247" and you get the typical six thousandths loaded slop. Cram .227" bullets in there and make it four thousandths, still loosey goosey by precision cast standards. You can still make them shoot, though, it just requires a little different approach from stuff it to jam length and pray.