Last thought...
Speer has been making a "short barrel specific" 38+P load for a while. The round is specially designed to maximize performance out of a 2" barrel, and has received good reviews.
Hope this doesn't get me in trouble...
http://www.speer-ammo.com/products/short_brl.aspxAnyway, this is the next load I want to try in our 642's. I would have done so earlier, but it has taken me a long time to burn through the ammo I already have (since these are NOT fun guns to shoot).