Your assumption about optimal barrel length probably isn't 100% true anymore. Some makers are using slower powders in .22s (such as the Winchester Super and Hyper Speed loads, CCI Velocitor, Stingers, Aguila Super Max,etc). So I don't think the old 16" rule is quite as uniform these days.
That said, even the 29" CZ Ultralux 452 doesn't slow high velocity down to subsonic, and that's about as long as I would consider practical.
What do you want it for: the accuracy of subsonic loads, or the quieter shooting?
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