I practice with 148gr lead wadcutters from www.mastercast.net. They are very accurate, and only beat my hand up a little when I fire them from my 637 Airweight (the twin to your 642 with a visible hammer). I limit myself to fifty, and first put a J&J knuckle bandage on the web of my right hand. If I forget to do that, I will make -- and break -- a blister within twenty rounds. Practice with Airweights is a duty, not a pleasure. My SD/HD load is the Remington +p LSWCHP, the FBI load. The Winchester and Federal versions of this round use hardcast bullets, which probably will not expand from a snubby. The Remington load uses dead soft lead bullets which can be expected to expand from a snubby. I occasionally fire off a cylinder full of these cartridges, just to remind me of how stout they are. If I were to use them in a SD/HD situation, I doubt if I would even notice that my hand hurt (or that my ears were ringing). The Speer 135gr +p Gold Dot is also highly regarded, and is made specifically for snubbies.
Cordially, Jack
Cordially, Jack