When Krista stabs Billy's hand with a letter opener, this would likely have caused serious nerve damage and noticeably impaired the functioning of Billy's hand. Billy's toned down reaction to being stabbed through the hand, instead of backing away, is extremely atypical, and it could indicate that he already has nerve damage (numbness) in that hand. However, if that were the case, then Billy could not wield weapons the way that he does. After being stabbed through the hand, Billy seems to have virtually no pain or impairment in that hand, and he continues to use it more or less the way he always has.
When Frank is showing Rachel how to disarm a gunman, he refers to an empty handgun magazine as a "clip." The correct term is magazine, and with Frank's history and experience, he should know the difference.