You just need Dogmeat to obtain the weapon, you can find him at the Red Rocket Station.

And here is your doggy!

You need to head back to the Vault 111 now. (there is no need to show it on the map lol)
Here's the weapon locked behind an hard lock and since you are level 1 you cannot pick it up.
Talk now to Dogmeat and ask him to find any items in the area he will bark and he'll come back with the weapon in his mouth.







Now trade with him and he will give you 200 bullets for the weapon!

That's the proof that I was level one and I did not pick the lock.
I hope that this guide was helpful for you and have a great time playing this game!