Another thought ... are you sure these bullets are designed for .38/.357? Quite often, FMJ bullets in this weight range - and w/o a cannelure - are intended for either 9mm or .38 Super. This means their diameter would most likey be .355/.356 - versus .357/.358 for the .38/.357 revolver rounds. That may not sound like much, but if you are already rather aggressively expanding your brass, slightly undersize bullets could be causing you grief.
Pull out your micrometer or Vernier caliper and check a few.