Like I said: it needs a lot of material removed to fit and float properly. I used a dowel with sandpaper and it took forever. Now that I have a mill, my next one will probably be done with a 3/4 ball nose end mill and just do the finish work with the sandpaper dowel.
FWIW, my experience with 178 a-maxes is that they handle the sonic transition pretty well. With their secant ogive, though, they don't jump well in a long throat like you'd have in a factory barrel. I switched to 175 SMKs because they don't get unstable with a lot of jump. Of course, all my shooting is only at 600 at the ranges in my area so I don't have to worry about the sonic transition anyway. I'd still discuss various chambers with your smith before he cuts your blank.