This is a similar problem as @zeke1220's issue which was closed as resolved in January: https://feedback.bistudio.com/T156331

The Magnum has a wrong initSpeedMultiplier of 0.9 and sometimes takes 3 chest shots to kill, instead of regular 2.

Muzzle_Base initSpeedMultiplier is 1.0 and Second_Muzzle inherits.
Second_Muzzle initSpeedMultiplier is 1.0, and all other muzzles inherit.
But Magnum initSpeedMultiplier is 0.9.

So when firing, sometimes Magnum deals intended damage, sometimes Magnum deals less damage depending on the muzzle/chamber. Example server log:

19:05:47 | Player "Unknown/Dead Entity" hit by Player "Survivor" into Torso(16) for 48.7726 damage (Bullet_357) with Revolver from 22.1107 meters 

19:05:47 | Player "Unknown/Dead Entity" hit by Player "Survivor" into Torso(16) for 54.1918 damage (Bullet_357) with Revolver from 22.1107 meters

You can see 48.7726 / 54.1918 = 0.9

All the muzzles and the Magnum class itself should fire with the initSpeedMultiplier=1.0 for the ~56 damage and 380 m/s.

(please take care to avoid the initSpeedMultiplier=0.9 which makes the Magnum a 3-shot kill like a 9mm. This was probably added in error as several other weapons were given an initSpeedMultiplier=0.9 in today's update.)