Corrected entry: Just before the sting goes down, the assassin and the bodyguard are shown preparing their weapons. Both place largish suppressers (aka "silencers") on their guns. Both of them are using revolvers. Except for a few, rare models built specifically for the purpose, revolvers can't be effectively suppressed. There is a gap between the cylinder and the barrel that allows some of the expanding gasses and accompanying noise to escape before they can be affected by a suppresser on the end of the barrel. This is one of the most common firearms related mistakes in older and period films.
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Correction: The key word is "effectively", from the submitters description it appears that the silencers offer some noise reduction which is presumably enough for their needs.
tw_stuart