The trick to eliminating leading and improving accuracy lies not in using a harder lead bullet. A "soft" bullet of, say, bhn 9-12 will work wonders in those regards, as well as expand reliably if driven 800fps and faster. Size the bullet for a slight interference fit in the cylinder throats - ignore barrel groove diameter - and use a good lube. (Hot powder gasses blasting past the too small bullets is what causes leading in throats, forcing cones, and bores.) I know that going softer is counter intuitive, but a soft bullet will obturate to fill out and seal those gasses in, and a hard bullet won't.
Those Speer lead SWC's are good bullets, but they make them on the small side so as to work in all guns. (Yes, Virginia, there was/is a large discrepancy in throat diameters over the years. It's why I keep .356, .357, and .358 diameter bullet sizing dies handy so as to match my bullets to the particular gun.*) That, plus the minimally efficient dry lube they use, is why the Speers aren't the greatest choice when working up loads at the upper end of the scale.
*I have three .38's I load/have loaded for: a 40's vintage Colt Officer's Model Match took .356's, a 60's vintage S&W K-38 took .358's, and my current 1949-vintage S&W K-38 takes .357's.
Last edited by gnoahhh; 12/10/20.
