This cylinder is fluted. Those grooves are cut into the cylinder in between chambers to reduce weight, and there are as many flutes as there are chambers in the cylinder - in this case, there are six.
Photo © Russ Chastain
Fluting is the practice of cutting longitudinal rounded grooves into cylindrical objects in order to remove material and thus make the object lighter.
In guns and firearms, fluting is most often done on revolver cylinders (as in the accompanying photo) or barrels.
When it comes to cylinders, the fluting is placed so that it lies between the chambers, in order to preserve the strength of the cylinder material around each chamber.
Unfluted cylinders are heavier, but are stronger.
Fluted barrels allow for greater rigidity than thinner-diameter barrels, but without quite as much weight as large "bull" barrels.
Fluting also adds to the surface area of a barrel or cylinder, which helps it to cool a little bit faster.
- Russ Chastain