Relatively early in the main quest.. (disclaimer, your definition of relatively may differ from mine) you can get what's possibly the best sniper rifle in the game.. the Ratslayer, simply by wandering into the right place. You have to be willing to explore slightly. Its a unique Varmint Rifle with scope and silencer. Combine with Hollowpoint or Armor Piercing 5.56mm depending on the target
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with a bit of crouching for the sneak headshot critical. Once your skills are beefed up a bit it's good enough for long distance 2-3hit Deathclaw kills, one to the head to start, followed by 1-2 more to the Deathclaw as it heads straght for you.
There's also a quest later on that sends you into the relavent location.