New from Marui, the "Perfect Hit" BB series are actually about 5.95mm in diameter and made with the very same process as their bearing manufacturing. This makes the BBs slim, consistent and high quality enough to be used in tightbores as low as 6.01mm as suppose to other BBs which might vary enough in diameter as to cause inner barrel abrasion or jamming.
At 0.28g, these BBs are best used when accuracy and follow-up shot consistency is a larger issue then absolute FPS. At this weight, the BB will travel slightly slower on the same spring then a lighter pellet but it will be less effected by cross-wind and turbulence and so it will land slightly tigher groups but more significantly will follow more consistent flight paths. This is important for bolt-action weapons which rely on the previous shots to optimally place follow up shots.