If you're referring to fluted bbls. this photo shows a fluted heavy bbl.
This is done to heavy barrels to lighten them & improve cooling. This decreases rigidity, compared to unfluted heavy barrels (The result of reducing cross-sectional area), by approx. 20-30%, but the increased cooling performance more than compensates for this. The big issue w/ fluting it is that stresses are introduced to the barrel during the groove cutting process. If done properly, the barrel's accuracy & grouping will remain pretty constant. It's also important that the grooves not be too deep, or else pressure threshold will be dangerously compromised. Seems that a fluted barrel should be cryonized to properly reduce those stresses.
Personally, I favor them (if done correctly) because they strike the best compromise between strength & light weight. They offer increased cooling performance over full heavies & are stronger than light bbls., being only about 50-60% heavier. They are a good idea in a M4-type, heavy bbl. carbine/CQB gun, but I wouldn't do it to a sniping rifle, where the added weight of a full heavy bbl. is actually beneficial. Overall, for the right type of gun, I think the benefits outweigh the deficits. Plus, they look bada$$ too
. I will be swapping out the barrels of 2 of my own M4geries for these.
Revolver cylinders/wheels are also commonly fluted. The flutes are the grooves on outside of the cylinder. This is probably done to lighten the gun a little, & to enhance gripping the then cylinder when loading.