I realised this after looking at this video after watching the Heavy is Dead long time ago: The weapon that was used by engineer looks like a gun called the Heart Attack gun. It looks like a Deagle with a laser attached to it and it was supposed to replicate the stun gun. The reason its called the heart attack gun is because it causes symptoms that are known as heart attack. The gun is also mostly seen in the game as a lethal weapon in a game of Call Of Duty: Black Ops Cold War. The gun looks like its bullet is replaced with really lethal bullets. Its a scorestreak and more similar to the annhilator