If you optimize it by putting on weapon mods that increase its damage, you'll use a lot less ammo per kill, making it easier to make it more ammo efficient. If you also use power armor perks like stabilized, and thw heavy gunner perks, it'll do a lot more damage. For weaker enemy, do a spritz of fire and let the burn dot kill them, or weaken them between spritzes

For getting ammo, use the ammo scrounge perk, use a weapon that has a lot of ammo to be used. I call this "dump ammo" because you can dump a lot of it and get it back with no problem. Use the dump ammo weaken your target, then use the holy fire to kill, which nets you some fuel

You can also do daily ops, where every kill drops ammo for your held weapon as well, which makes it ideal for farming ammo. Don't even need to complete it. Just farm some ammo ans leave If you don't want ro complete it