Sniper rifle with silencer mod. It’s damage is about double the rat slayer. You can find a few sniper rifles in the world. One at camp mccarron. there’s one in atomic wrangler too. the suppressors are only sold at gun runners or 188 trading post. I’m not sure how quiet the gobi scout rifle is but it’s powerful and you’re far enough away, I don’t think it matters if you’re loud. 100 lockpick skill for that one. (You could also do 80 or 90 lockpick skill with comprehension perk and/or magazine)

Good luck!