Yeah..... we're not talking about an anti-Castro Cuban trained to assassinate Fidel, or a mafia hit man, or an assassin for hire from Marseille, or a military trained assassin, etc. but a hypothesized guy with a junk weapon and no experience or training other than Marine basic training (assumption). So, I agree his probability was probably lower, but the above is a conservative estimate w/o that assumption.
btw - the prob(success) on the deformed bullets should have been
total successes, likewise on the prob(failure) .... sorry about that.