Mobuck said:
I'm just guessing that the 22mag is still burning powder at the muzzle of a 16.5" bore. Based on the amount of muzzle blast from a shorter than 20" barrel on 22mag and 17HMR.
That is absolutely correct. In fact, I have chronographed .22 rimfire ammunition ( over an Oehler 35P ) out of a Winchester 67A with a 27" barrel and the velocity exceeds the printed, and box velocities that manufacturers list.
A few years back I contacted several .22 rimfire ammunition manufacturers asking what barrel lengths they used to achieve the printed velocities they advertised. All, at that time, stated that 26" barrels were used. Since that time, most have gone with 24" test barrels, or rifles with 24" barrels.
I have the return emails from all that I contacted, except for the one sent to Winchester.
Now, do the velocities increase past the length of a 20" barrel? Yes, they do, but it's not by any GREAT margin. Still, the burning gasses are still working quite a bit past the touted 16.5 inches. Only 12 to 14 inch efficiency is a joke.