I think you're just stuff at the game.
Ammo scavenging perks early on grant you more than enough ammo.
Swap between weapon types to prevent burning through one ammo type too quickly.
Purchase ammo from shops, selling unused ammo.
Get Barter perks to reduce costs, and use Orange Mentats.
Sell pre-war money or cigarettes for a good flow of caps, or get Cap Scavenging perks.
Improve damage through relevant perks and modding weapons, to get the most out of your ammo.
Basically, get good, noob.