First get the holotape, a note from Little Moonbeam's father (from Statesman Hotel).

Then the go to the national archives and do the quest Stealing the Declaration of Independence.

After that a woman called Sydney will be in the Underworld, ask her about herself, then say that she has a nice gun.

Then exit speech and ask her what happened to her Dad, then you tell her that you have a holotape from her dad.

Repeat this as many times as you want you and have infinite ultra SMG's and 10mm ammo, all you have to do now is sell them for infinite caps.