Quote:
18.6/70This is the chamber diameter, 18.6mm which equals .0632", the diameter of a 16 guage chamber. 70 is 70mm chamber length.
16.5/17.116.5 is the diameter of the choked portion of the barrel16.5mm which equals 0.649". 17.1 is the diameter of the 16 gauge bore17.1mm which equals 0.673". The degree of choke is 0.6mm or 0.024". This equates to an improved modified choke.

um......how does 16.5 mm equal 0.649 inches while 18.6 mm equals .0632??
18.6 = .732 which is a 12 gauge nominal bore size, 70 is the chamber length (23/4")