When you first arrive at the front gate there will be a child that will take you to Dave, after you finish talking to Dave, kill him and take his key and use it to open the safe right next to him. In the safe there will be a rifle that is called the 0l'painless. the rifle does 21 damage but it's way better than the sniper and a lot easier to repair and find ammo for it.
Achievements
Vault 101 Citizenship Award (10) - Got the Pip-Boy 3000.
The G.O.A.T (10) - Took the G.O.A.T.
Escape! (20) - Completed "Escape!".
Following in His Footsteps (20) - Completed "Following in His Footsteps".
Galaxy News Radio (20) - Completed "Galaxy News Radio".
First go to RockBreakers Last Gas and take care of all the Enclave. Next look up towards the cliff and you will see a shack on top. Climb up the hill and enter the shack. Right to the right of the door there are two radroaches in a cage with pet names so thats pretty cool. In the locker in the back there is a sniper rifle called the Victory Rifle. It deals a great deal of damage and is the best rifle in the game so I highly recommend it. You need a lockpick of 100 to get in it.
Abe's Rifle
In the lower halls of the Museum there's a room with a turret shooting at feral ghouls. If you head up to the room with the ghouls in it, look near one one of the walls. There's a display case with Abe's Rifle in it (it's really strong. It uses .44 shots)
The Terrible Shotgun
In Evergreen Mills (near Casey Smith's Gas Station) there is a guy called Smiling Jack, kill him and you will have The Terrible Shotgun.
When fully repaired it does about 80 damage. The only bad thing about it is that it degrades fast.