Mafia 42. Half-Life Period

Although the machine gun is fast-firing, its "large black barrel" doesn't quite fit the description. It must have been a machine gun.

Generally, when you first "wake up" and see an object up close, it always seems larger than it actually is. It's an effect of surprise. So, a machine gun could also be a possibility.
But still, there are no killings attributed to the three mafia groups. While Rasa X can be attributed to Koshi's death, what about the others...?