Don't the BFTA rules state that a gun should not be modified during a shoot. If so, in the same way that say, a butt hook, cannot be attached or detached during the shoot, then neither can a chrono. If the chrono is attached before the shoot then it may not be removed during a shoot,or vice versa.
Having said that, it's my own belief that reference to a chrono during the shoot and consequentially acting upon the result is not within the spirit of the competition. I know that the BFTA rules state that a gun may be chrono'd during a shoot but I contend that this is carried out by a marshal to ensure compliance with the rules and the law, rather than as a test of the efficiency of the equipment.