I have not tested this yet with other weapons but i know it works with a Disintegrator. you will also need to have unlocked the Lets Pretend in the old area of the Luchadores ( check your map if you have not played saints row : The Third.)

(UPDATE: Any weapon can be used, but the disintegrator and Energy sword were most effective)

after unlocking the Let's Pretend, go to the wall opposite the door and to the left of the checkout. there should be two doors: one with a mop in front of it and another without

After going in front of this door, get out your Disintegrator and shoot the left door on the opposite wall to the entrance. it will evaporate and essentially open. This will reveal a staircase that you need to go down. go down the stairs and you should see a large rooms, filled with headshots pictures of all of the developpers. to the left is a bathroom with a sex doll on the toilet, and to the right is a small chair. on the chair is the tiny pistol. Pick it up (note: it is a pistol shape and size, but takes up the RPG slot). The gun has a charge. it uses 25% to shoot and recharges slowly. I would recommend that you were to go to the nearest friendly fire and upgrade it.

thank you for taking the time to read this