Just look up a fractional inch to mm conversion chart. Then break out the drill bits.
5/64 inch = 1.9844 mm
3/32 inch = 2.3813 mm
7/64 inch = 2.7781 mm
a 5/64 bit will fit the 2.1mm barrel but not a 3/32
a 3/32 bit will fit the 2.5mm barrel but not a 7/64
