Talking about the wood on the rifle could warp when exposed to moisture, and how there always lots of moisture when he used; as in, in the jungle.

>I had the feeling that the period of his life when he became
>familiar with the gun was a period he would rather not relive.

And to going to war with the Juarez cartel would have a lot of similarities to go to war in Vietnam. Him putting his life on the line for a person/cause that he doesn't believe in. And he'd be potentially putting his granddaughter in danger.

For Mike, at that moment, it's probably one thing to kill someone who was involved in the death of your son, and it's something else entirely to kill for the money and the "glory."