Hmm... My new DE50SRMB has the same ~0.3984 measurement (as measured with $30 general purpose digital calipers). It ran flawlessly through 60 rounds (using 3 different mags) yesterday at the range, but this piston clearance issue still makes me nervous. I've only tried it with Underwood 300 gr JHP, which is on the warm side at a claimed 1580 fps.