I'm pretty sure your Model 39 has a tighter bore than the Beretta and that's why it is more accurate with the .356" bullets. My Beretta shoots cast bullets accurately, as long as they're .3575" to .358" in diameter. I don't know about other Berettas, but that may be the case with them as well. The large diameter bullets I use in the Beretta also do well in a Sig P226 and a Walther P4. I use Bullseye powder with a 150 grain round nose (Lyman .38 Special mould).