Here, the pronominal form is to express that it's a reciprocal activity. I will shoot, but I know they will too and the speaker is making sure you know he's not afraid of receiving some shots himself.
I will let someone else check on this, but to me, the "I" in the song changes from stanza to stanza. First, is the
rielera speaking, then it's Juan.
Juan is the guy with the guns, ready to use them if he gets into a fight with the others in railroad related jobs (that's how he brave is!).
He talks about having bullets for his sweetheart and the guy she may cheat on him with. And at the end we know he's a
rielero as well.
"Garrotero" is a helper in the locomotive. They pulled the brakes (
garrote) and they still perform other activities the train driver (
maquinista) orders.