The entrance to the Dino Bite Gift Shop is located on the left side of Dinky just up the small flight of stairs. Inside the Dino bite gift shop are a shelves with various items on them and the door to the Dino Bite storeroom. You can steal the key to gain entry. Inside the storeroom are approximately 160 rocket souvenirs, and 200 Dinky the T-Rex souvenirs along with a Sunset Sarsaparilla vending machine. (The storeroom is irradiated most likely because of the rocket souvenirs). That Gun is on the shelf to the right in the storeroom. There is a flight of stairs that will take you to the mouth of the Dino. Manny or Boone will be outside on sniper duty depending on the time of day.
The storage room behind the shop counter is radioactive (1 rad per sec). Even if you take all the rocket souvenirs, the room is still irradiated.
When you ask Cliff Briscoe whether he sells guns, he states that no one ever wants the dinosaurs; this is confirmed by the shop inventory. It is possible to buy 1000 dinosaur replicas from him, and you can find many more inside the storage room.