I converted my Webley Vulcan to a leather seal over thirty years ago; it's still on the same one, and is smooth and very consistent. My Ospreys are similarly converted.
As Hsing-ee says, leather is very tolerant of cylinder wall irregularity, but they need to be sized to a good fit. Usually a relatively loose fit works best, it should be very easy to push it down the cylinder. Too tight will rob it of power.