Remember, the most commonly used weapon in mafia hits & assassinations is the .22LR, not some huge caliber.
A .22 will kill you grave yard dead. How much deader do you have to be. And it is not all that loud. Fire one shot, they can’t tell where it came from. Fire two or three and they can start locating it.
Economical, too. Easy to imagine that a person so inclined during Clinton administration could buy a brick a month on shopping trips into town Walmart; and in 25 years time, would have a mighty impressive stockpile. Quiet. Economical. Effective.