It's been awhile since this has been talked about, but I think Chief recommends nothing under 18". I do plan on chronographing the difference between a Volquartsen 16.5 THM barrel and a Volquartsen Snake fluted 20" (Actually 18.5 w/o the comp) barrel.
I have looked into this before and alot of people use the 22LR data to answer this question but I have seen a number of chronograph figures that will dispell the 16.5" or 18" barrel theory. I stand with no barrel shorter than 20". Look at the thread below marked "Velocity Test" and you will notice he is shooting a 24" barrel, 24" -18"= 6", if the velocity loss is 12 fps per inch then he would have lost 72 fps, which he may have. BUT with his velocities listed like the Rem Premier velocities listed at between 2100 and 2200 fps I doubt that he lost 72 fps. The Rem premier is factory listed at 2000 fps. All barrels shoot different, faster or slower and some has to do with the chamber and quality and tightness of the the barrel. There is a velocity chart floating around that shows 17 hmr barrel trimmed one inch at a time and it shows the max velocity at about 18.5" or 19" but that was not a 22 wmr barrel cut down and multiple ammo tested. The powder charge in most 22 mag shells is between 6 and 7.5 grains whil the charge in a 17 hmr is only about 5.4 g.