The idea that 22s slow down after 16" gets thrown around a lot but the bottom line is that different types of ammo will perform very differently and individual barrels are not all the same. For some rifle and ammo combinations 16" is ideal. For others, it is not.Lesson 2: barrels longer than 16" start to slow down the bullet.
I have heard this bandied about over the years, but chronograph tests will negate this theory pretty fast. As 22LR bullets leave the case, they gain velocity very quickly for the first 6"-10", then the velocity gain curve slows down beyond 16"-18". As you approach 22"-24" velocity gain per inch of barrel is minimal, and the variation in velocity is also reduced (which typically increases accuracy/consistency). You would have to have a very long barrel before you would notice any loss in velocity, but it could happen, probably past 26" or so depending on the ammo. As long as the pressure X bullet base area is greater than the friction X surface area, you will continue to gain velocity, which is why the gun goes BANG, although with longer barrels there is a substantial reduction in report (indicating a drop in pressure differential).barrels longer than 16" start to slow down the bullet
Lesson 2: barrels longer than 16" start to slow down the bullet.