The term "stopper cartridge" refers to the "dang big gun" that a professional guide would choose to stop a charge from dangerous game such as Cape Buffalo or Elephant.
Think 416 Rigby or heavier.
The ivory hunter Wally Johnson reported a Cape Buff that took 5 shots from an assistant's 9.3x62 before he ended it with his 375 H&H.
So that is why it isn't a "stopper" like the big bore safari rounds.
Machine guns are awesome until you have to carry one.