I've got a 640-1 that is Mag-Na-Ported. While it does cut down on muzzle rise a bit, the concussion is significantly magnified with full power loads, and at night it is completely blinding. That said, I find it quite comfortable to shoot hot 38 loads. I would assume that Mag-Na-Port could handle your 340. They'll port the barrel and shroud separately like they would a semi auto slide/barrel.
- a quick google search found this Lessons from the Plate Rack