DON'T! Everything I've read says this is an extremely dangerous combination. Yes, it would make a 642 go into KB mode.

Dimensions are all different and the .38 Super has a SAMMI max pressure of 36,500 PSI as opposed to a .38 Special +P max pressure of 20,000 PSI.

You might succeed with .357 Magnum chambered revolver but why risk it? Just put FBI +P loads in the 642 as they will stop anything on two legs that a .38 Super would stop - and do it cheaper.

William