Time to 'fess up...... Many moons ago I was machining an S200barrel in order to fit it to an S400 action. When fitted, I could only get the muzzle energy to around 6 f.p.e. On investigation I found that I had been one revolution out on my metric 0 - 25mm micrometer and had machined the groove that the air travels around from the transfer port and had machined it at too small a diameter. There was enough meat on the barrel to part that off and start again.
Neil