Hollie's explanations of "I dare ya" and "serving twenty" are perfect. I would add that "You'll get your head blown off" implies that someone will shoot you in the face/head.

This language is not friendly at all and is, in actual fact, threatening. "I dare ya" The word 'ya' is a slang term for 'you' so it actually means "I dare you." This phrase is used when the speaker is challenging the other person to do something; usually something they don't want to do. It is a taunt. "your head blown off" This means that the person's head will be decapitated completely. "servin' twenty" This is meant to be "serving twenty" and it means to serve twenty years in prison. "All of them are going down the same road either dead or servin' twenty." This means that all of them will be dead or in jail with a twenty year term.