I can only go by my years of experience shooting both, and on "average", i'd say when it comes to over the counter center fires, it would be revolvers that are more accurate... BUT, when it comes to rimfires, the revolvers have to be compared to the likes of "over the counter" High Standard Victors, and S&W M-41's, so in rimfires, i give the edge to the semi auto's.
My S&W M-18 is very accurate, but it doesn't hold a candle to my Victor.. In fact my Victor will outshoot any rimfire revolver i've ever owned or shot against, and probably a big percentage of the 22 rifles sold today...
DM