IF you have a good build and use some tactics you are able to conquer with NPC Faction like NCR Army, Hub Patrol or VC Patrol.
They have not any inifinite amount of ammo - you just get kill before they shoot at you all of their ammo

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However it's logical that the best way to fight with above mentioned NPC's is to attack them with 3 players at least, because 1 character will not be able to carry all of the loot from one encounter.
Backing to the topic - change your build or find some mates instead of crying on the forum.