Every gun can be a bit different and it was mentioned in the handloader article. Most of the 38 Supers I have had much prefer .356" diameter bullets over the much more common .355" bullets. All the factory ammo I have tried used .355" bullets.
The HAP bullets that Borchardt mentioned are .356" bullets and shoot much better in my pistols, there are other .356" bullets that shoot well also.
I had a Barsto barrel in my Super, and they told me the bore size was .3565", and indeed it shot better with my cast bullets better than any 9mm jacketed bullet I shot in it. It was a dandy gun (I bought it when Colt was switching to the Series '80 guns, it was left over and I got it on a real deal back then). I built it as a field gun, I had some old Micro sights cut into the slide, did the beavertail myself, and had a buddy do the Clark TigerTooth stippling on the front strap. I had a lot of fun with that pistol.