I love my P35s but the 25, in particular, won't push pellets as fast as I want. I added an after market plenum which helped but my velocity on 25.4s with it's current tune was about 840 fps. Usable but not great. I saw the oblong barrel ports on some more expensive guns and decided to try it. I first used a drill bit, I think is was a number 3 to take off some buy rocking it along the barrel length. You don't want to go bigger than about 85 per cent of bore diameter and I drilled the entire passage from the valve to the chamber to 85 percent previously. After that I smoothed the opening and elongated it a little with a diamond grit ball in a Dremel type tool using lower speed for control. I was careful not to hit the other side of the barrel drilling or with the grit ball. I did it in two phases and the pictures are of the first phase where I measured the opening at 5.7mm wide and 6.2mm long. That's wider than the drill bit but I ground a little on the side to address a mark from the drill bit. Later I came back and made the hole about 6.6mm long. Afterwards I replaced all the barrel O rings. On the same barrel clamp chronograph I measured 853 fps afterwards, a gain of 13 fps. On my tripod mount chronograph I got as high as 858. Small change but no real cost. The picture with the barrel vertical is before and the horizontal barrel is after.
