Afternoon dan01

All else aside like chamber tightness or barrel quality an 18" barrel on a .22 does 2 major things for you.

First, if you are shooting iron sights then the 2" longer sight radius is an advantage. If shooting a scope then no advantage to the longer barrel in the sighting area.

Second, if shooting subsonic rounds it gives a bit longer barrel for the gas to subside before exiting the barrel so that extra 2" can take a fair amount of noise out of a non suppressed lower velocity shot.

As far as accuracy goes that depends on a lot of things including barrel quality, rifling, chamber shape, & barrel harmonics. In some cases a longer barrel can even decrease accuracy if the barrel & rifle isn't a tuned unit.

As far as velocity goes.. I haven't tested all the possible combinations of ammo & barrel configurations but as a rule from what I have seen in my limited testing is that a 16" being the fastest for most over the counter .22 semi-auto configurations. Not much loss with an 18" barrel (but definitely not faster) & a measurable loss in velocity with a 22" barrel.