I have an Impact M3 with a huma regulator. I have a slow leak between the reg. and the gun body. Takes about 4 days to leak down from 250 bar to 230 bar. I can find no charts, diagrams or tables that give o-ring sizes. I suspect the manufacturer wants you to buy their rebuild kits instead of just the needed pieces. Anyway, I need the large o-ring that seals the gun body and the large flat surface of the nut. I measure 14 x 2 with a ruler but my old eyes aren't as sharp as they used to be. 