You are right mate we use 12mm bells the same as on cat collars and are very good, my mate can hear them lovely unfortunately I can't. I have ring in my hears all the time, some 40 years ago now somebody fired a 12 bore next to my ear when out lamping. I think there is still a risk when your doing a hedge with a lot of cover a burrow 100 meters or so long and only 2 handed we will pick and chose till it dies off a bit more.
Cheers Arry