Having used the old 30-40, I can say that it is one fine caliber, and easily able (good shooting) to down most antlered game. The only reason I went to a larger 35 whelen was to stop chasing elk. Larger hole and mass. But the 30-40 should be a sweet and handy DR on a 20 gauge. Both Birdhunter (Bob, and Ellis Brown know much about these type of conversions, and their advice would be most beneficial.