The "teeth" keep the cap from unscrewing.
There are two types.
The early type as used on the Wingmaster and Police guns have machined "dimples" around the outer diameter of the cap, on the inside.
These work with the barrels that have the spring-loaded plunger in the front face of the barrel support ring.
The second type is the Express type.
This one has machine-raised "burrs" down inside the cap and are used with the Express guns that have the plastic magazine spring retainer with the coarse "teeth" on th end.
The two are not interchangeable. If you use the wrong type, the cap will vibrate loose, and unscrew.