Ok so I undestand that the gun called "This Machine" is an M1 garand. It's awarded at the end of the quest "dealing with Contreras."

So I found him and started his little missions. The first is to give a note to Issac at the Gun Runners.

Do I actually have to lockpick my way into the gun runners? Or what?

his house is in the area near the NV medical center in that small housing area

nope. isaac should be there at the vendor with the robot in it. if he isn't he disappeared and you can try 2 things:

1. keep fast traveling to gun runners. if he doesnt spawn try:

2. walking east from the vendor. if you keep heading east you should find his house. go in and out again. you should then see him running to the gun runners vendor. follow him and then start talking to him.

hope this helps!

Seeing as the OP is looking for a hint, discussion of which involves spoilers - I'm going to go ahead and move this to the "Cheats, Hints, and Spoilers" forum.