You buy the size closest without going over. You can use a lathe to bore it to size, or you can make a mandrel for which to forge or stretch it a little through the use of red heat and a hammer. I prefer the lathe method myself.
Malm,
When you use the lathe method are you using a 3 or 4-jaw chuck? One would also presume that the ramp portion of the banded sight is facing inwards and not out.
