I'd say his chambers and throats need a good scrubbing. dahermit is right; there isn't a manufacturer that purposely makes the throats too small in order to prevent bullet creep. By the same token however, if his chambers and throats are SAAMI spec and his bullets are indeed .357", he shouldn't be having this problem.
Pushing cast bullets of known size is a standard way of checking to see if chambers are reamed to the correct size, and suggested by many cast bullet shooters to pre-determine what size the cast bullets need to be (some cast bullet experts advise sizing the bullets to fit the chambers, not the barrel groove size), before a lot of shooting because as stated, there is a problematic relationship between chamber size and forcing cone/barrel size.
Some years ago, many Ruger Single Actions were being shipped with chambers smaller than the throat of the barrel. The bullets therefore, were "sized down" by the chambers and entered the barrel throat too small, casing sever leading and inaccuracy. Over the years this problem has been posted about over, over, and over. The original poster was likely alluding to that problem when he posted.
Several who answered him obviously did not have the background, history, experience, to recognize that issue. But, boy they sure do want to post something about it... or anything. Goll darn kids!
We old timers do not know everything, but usually when one has experimented, read everything we could about the subject, and studied the subject, since 1963, we know quite a bit and we get short with some kid who, like the old saying, two ears-one mouth, should listen twice as much as they talk (post).