Well actually for a 22, there is some data that would suggest a longer barrel is not more accurate. My understanding is that if barrel length isn't needed for the extra muzzle velocity it does not help accuracy. The Bullet Dwell time will be higher in longer length barrels. The time the bullet stays in the barrel is longer and as a result, you have to keep the action and barrel completely still for more time compared to a shorter barrel. That is why you see some tactical rifles in .308 and .223 with shorter barrels among other reasons like weight and barrel rigidity. If it were me personally I would go with the 18" barrel. It will be more than you need and your not going to see that much of a difference in the muzzle velocity of the 22lr between the two.
Based on my experience only, there is no advantage in the extra length. 18" performs and handles the best.
If you google it there several articles out there but they quickly get into math and detail that make me suffer.
Let us know how she shoots!
With target/std and even some high velocity the shorter the better IMO. There isn't an appreciable gain in velocity past about 12-16", the extra length can provide additional radius for open sights and maybe balance if you have a heavy stock, but for most it just strips off more lube, makes CF/aluminum less rigid and makes the rifle less agile.
I've talked to the folks at KIDD and Volquartsen and they agree that the longer barrel has the highest accuracy potential.
Did they give any scientific reason to back that up?