I'm no expert, but I'd have thought that injecting someone in the mouth is a good deal more difficult/risky than injecting them in the arm.
Once they've shown a dentist the correct part of the arm to inject, it's hard to see how the rest of the seven hours and fifty five minutes of training is allocated.
Well dentists sure as heck have scarier needles. Maybe they need the extra lesson time so as not to frighten the sh#t out of the patients!
It’s funny you mentioned the darts players, I used to be a nurse and I remember my training at uni we were given a sponge with a plaster on it, told to hold the syringe like a dart and just stab! Okay maybe there was a little bit more to it than that but no surprise, you don’t really learn until you have a real patient in front of you and then you find everyone is different. But clearly, darts players will have a head start in their training but maybe you’d need to breathalyse them first as they usually have a pint in the other hand to steady their nerves!