I don't know that I've heard that "ammo will always be faster in a rifle". Certainly it is generally so. Equally certain, it is not always so. Some ammo is optimized for certain barrel length, either by design or coincidence, and will lose speed in barrels longer than that length. Usually the length that speed begins to drop off will be somewhere greater than about 15 inches with handgun ammo but I wouldn't be surprised if a rather low pressure round like the 22 will stop gaining speed well short of that length.
Still happily answering to the call-sign Peetza.
You do not HAVE a soul. You ARE a soul. You HAVE a body.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.