Dreadwitch
Just because someone fails that shouldn't mean they can't keep trying. Does that mean you want to limit how many times people can attempt to do anything?
There's a host of reasons it make take people a lot of tries, including disabilities!
Of course, I already said that there can be many reasons why people might fail - and it goes without saying that allowances must be made for those with disabilities - but I'm sorry, if you've failed a driving test (theory or practical) 75 times, then that has to raise some red flags, surely?
As Allira says, you're talking about someone being in charge of a lethal weapon, and if they keep failing so many times either through nerves or lack of ability in one way or another, then I'm not sure they should be in charge of that.