All I know is that since before I joined the forum, this debate has resurfaced regularly. Call it 8 or 10 years since I started lurking this forum and I bet you can find threads on it older than that.And in this time frame people have been posting an aweful lot of kills with both of the most common calibers.I have come to the conclusion that if the airgun is powerful enough and accurate enough, the caliber is purely a matter of personal preference. There are tradeoffs in trajectory for a bigger pellet at the low end of the power spectrum, but as the power increases you lose accuracy in smaller calibers driven supersonic.Other that that, just use what you like.
Smooth bore means unrifled.