It stopped raining here just long enough for me to get out to the range and try my new .357 Magnum barrel. The .357 test loads I had made in advance were a disappointment. They were a plain-base 158-gr wheelweight bullet over 6 grains of American Select. This is just below the top charge weight that Alliant lists for the bullet and powder. It produced poor accuracy and some key-holing. My standard .38 Spl load did just as badly (4 gr American Select under the same bullet -- just fine in my two .38 Spl revolvers). Anyone have any ideas why the bullets aren't stable? They seem to perform well enough in my revolvers. FAQ shows the twist rate at 18 3/4"; I'm not sure what twist rate S&W uses. Primers where not flat in any of the three loads tests -- more oomph might stabilize them better. It might also make the leading worse, though.
A somewhat hotter load did a bit better: 125 gr Hornady XTP over 6.2 grains of Hodgdon Universal. No key-holing, better groups.
When I got home and examined the bore, I found a good deal of leading just ahead of the chamber. I did not polish the bore before shooting, only swabbed it out good with Hoppe's and followed with a dry patch. Several applications of Shooter's Choice lead remover took care of most of the lead, and I followed that with mild polishing using a patch wrapped around an old brush and coated with Iosso polish. This is a non-abrasive cleaner, and it may not have done very much to smooth the bore, but it did remove the rest of the lead.
All loads shot low -- that one-piece front sight is too high. I removed the front sight and filed it from 9/16" down to a little less than 1/2". When I tried to replace it, the tiny screw that holds it on stripped. The screw seems to be some oddball size, smaller that the 4-40 screws in my parts box. Rather than send it back to NEF, I think I'm going to have the local smith open the mounting hole to 6-48 and just mount a Marble's orange bead or some other light-gathering front sight.
I realize that this is a completely unscientific report -- it was cold, windy and spitting rain yesterday, but I just had to get out and try my new toy. Hoping for better results after reading your comments.