Marushin. It's the main o-ring on the valve (ideally 1.5x5mm but was able to make do with 2x4mm). They always go for a softer o-ring, 50-60 durometer at a guess. While it's great for preventing leaks it opens up much slower resulting in lower fps which climbs back up after a few shots as the lowered temp causes the o-ring to harden. But considering it's only a 5-shot I'd want all those to be relatively consistent. Originally it starts 260fps and climbs to 290. Now it starts at 300 and goes down to 290 from cool down.