Most .22 lr ammo is already at 90% of its ultimate velocity before it has traveled the length of a pistol barrel. I would suppose that the powder is completely burned up before the bullet even travels a couple of inches and the remaining pressure continues to accelerate the bullet. At 15 or 16 inches, the pressure is so low that the bullet no longer accelerates and the remaining barrel just guides the bullet to the target.
The same thing applies to other pistol caliber rifles. I shot some of my .38 special handloads over a chronograph and the Ruger Vaquero revolver with a 5.5 inch barrel was only about 100 fps slower than the same ammo shot out of my Marlin lever action rifle with a 24 (IIRC) inch barrel.
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