Two is dead on the money. Bore size is .736.

Mike C.

 

Easy conversion of metric system: MM to English Inch is 25.4

18.7 / 25.4 = 0.73622047244 (.736 bore)

Same applies for rifle bullet diameter....6mm / 25.4 = .236

Multiply inch to convert to metric.... .308 x 25.4 = 7.8232mm

 

As others have said here, 18.7 mm is the bore size. It should also be the measurement at the begining or back end of the choke tube. So an 18.7 choke tube with a .027 constriction (full I believe), would be 0.708, or the front or end of the choke tube where the shot and wad exits. (0.736 - 0.027 = 0.708)
Now converting this back into metric, a full choke tube would measure 18.7mm on the bore end, or where the shot and wad enters the choke, and 17.98 mm at the constriction, or where the shot and wad exits the tube.