1) complete "reilly's rangers" . and find the "holotape to moonbeam"
2) complete "stealing indepence" and speak with sydney (she has to survive)
3) give her the "holotape to moonbeam" and she will give you her "ulta" SMG and 50 .10mm bullets.
4) the speech pattern can be repeated indefenitly allowing to receive unlitmited "ultra" SMG's and .10mm ammo.

NOTE:

it is recommended to get the holotape before reaching the rooftop of the stateman's hotel , although you can return at any time.

it is recommended that you give sydney the holotape right after comepleting "stealing independence" since on her way to underworld she has a knack of getting killed , and the option will no longer be available.

Her "ultra" SMG comes fully repaired , and is quite powerful. Its condition however , decreases extremely quickly , which is why it is recommended to have several copies of it (25+) , so instead of repairing it , you simply swap for a new one. Keeping the extra copies in your home.

Having a follower is also advised , as they can carry hundreds of pounds of this weapon , since the SMG only weighs 5 pounds you can easily get 50 copies totalling in 2500 bullets as well.

You can sell the extra copies as well , allowing you to have a substancial ammount of caps at an early stage of the game.