As a general rule many think a rifle chambered in 22LR with a 16½" - 18½" barrel is considered optimum for standard over the counter ammo. Everything in life is a trade off. As the bullet travels down the barrel it will gain energy by the explosive power of the charge behind it until a certain point where the barrel becomes too long and it starts to lose energy to friction. Another factor is the more length in the barrel the greater the stability relating to accuracy, but finally, a significant loss of energy results in an undesirable trajectory.
- too short of a barrel on any rifle and the bullet has not had the opportunity to be pushed fast enough by the explosive charge, also accuracy is less
- too long of a barrel and the explosive charge of expanding gas has weakened enough that drag from barrel friction starts to slow the bullet
It is that point where the bullet has achieved the maximum speed aka absorbed the most energy it can before the gas from the explosive charge has expanded to beyond where it can deliver any further energy to the bullet projectile, that is the point of the optimal barrel length. Anything longer is a loss. At the Optimum length the bullet should have gained sufficient stability while traveling at top speed. Additional length may lend itself to increased accuracy due to supplemented bullet stability however it will become an ever greater diminishing return to where the loss of energy becomes too costly to justify any further accuracy gained by barrel length.
Some shooters say that anything over 16½" is pointless, and even detrimental. This isn't always necessarily true. In match shooting you will see shooters using special subsonic cartridges in rifles with barrel lengths of 26". There's definitely a point to which additional barrel length is nothing but detrimental, however, a lot depends on ammunition and application.
My Ruger 10/22 has an 18½" barrel and it is considered a carbine. I've found it to be more accurate with open signs than some of my older more expensive rifles in 22LR with various barrel lengths. The energy lost to friction between 16½ to 26" is inconsequential as it relates to most shooters in ordinary application. It has more to do with the quality of the rifle and ammunition.
The drop on 22LR is rather ridiculous so I've never had much use for putting a scope on these rifles other than to experiment with ballistics while cutting down the walking distance between where I am shooting from and my target.