I used to take off the grip frame and barrel (so it does not look like a gun) and put it in checked luggage. The biggest issue is the bottle, they tend to frown on pressurized vessels aboard aircraft. So make sure it is empty, I used to remove the frangeable blow off valve, and again checked luggage. Worked over here never got questioned, but might be different over there. Just my $.02
remove the reg off the bottle, that's probably about it. if they search your bags then just say what it is - no license is required to own gear. but make sure its secure and not going to get broken as they do chuck your luggage about.