Just like the curved vials, these are made by injecting molten acrylic around a core inside a mold. The result is a curved hollow inside a straight tube. Unlike a curved vial, which must be set into a level in only one position, these vials can be read in any direction. Manufacturers like them because a single-barrel vial can replace two curved vials. Barrel vials are curved only on the inside.
Manufacturers can substitute a single-barrel vial for two curved vials.
Barrel vials, which are standard on extruded aluminum I-beam levels, have a quirk. The molding process creates a straight section at either end of the cylinder. And because of the molding process, barrel vials are more susceptible to slight manufacturing variances than other types. Both levels are at the same angle . And both are accurate.