ncbill
Thinks s/he gets paid by the post
A large-caliber firearm would make a lot of noise and be hard to conceal. Here's a story about a botched mob hit -- one of the guns found with the attackers is a .380.
https://www.upi.com/Archives/1982/08/01/Salvatore-Testa-son-of-slain-reputed-mob-boss-Philip/6836397022400/
Google "mob hit" and .380.
IMHO, if the character's a professional hitman they'd be using smaller calibers than .45 or 9mm, e.g. .22LR.
Probably with a suppressor & sub-sonic ammo.