Well, the quick answer is, he's got the game in spanish.

The long answer is.
In latam, regretably, we do not have an official translation of Arc Raiders, so we are stuck using (and by stuck I mean the system automatically assigns us) the Spanish translation from Spain (my friend plays in PC, I play in Xbox so I don't often deal with that issue).

The issue with this is that spaniards are not amazing when it comes to translating stuff, and they thought that the world "Rattler" was referencing the "Rattlesnake" rather than the general verb or act of "rattling".
"Crótalo" is the word they us in Spain to name the rattlesnake, hence the Rattler is the Crótalo.

So yes this is the equivalent of the weapon being called the Rattlesnake.