Width of receiver
Outer diameter of CO2 gas tube (where cartridge goes, see diagram)
Distance between outer surface of CO2 gas tube and outer barrel shroud (see diagram)
Outer diameter of barrel (see diagram)

I'm guessing he doest have calipers, which I recommend him to buy, makes it easierjrrdw wrote:You don't have your own to measure? Or yours can't be measured?; I'm guessing not but just to be certain...