After several missions (after "the car dealer" at least), you will have a mission where a man asks if you want some tequila. If you accept, he will tell you where you can buy it. Once you reach the place, you will notice that is a trap. Kill the banditos and get out. A text message will order you to go to "the guy who set you up". When you reach him he will be fleeing, but the car is so old that you can follow him by foot. Follow him, but do not attack. He will eventually get out to attack you with a modern assault rifle. Kill him and take the weapon. Repeat as desired.