343 Industries iterated through different Forerunner pistol designs before settling on the Boltshot for "Halo 4". One of those iterations was found Friday in the recently released Spartan Ops download for the Xbox 360.
The "Burst Pistol" was found in the files for a "Halo 4" Spartan Ops map by YouTube user TehBanStick who managed to access the game files where he found a very early model of the pistol. As it's name suggest, the pistol shoots a burst of projectiles at its target but it appears that it can do single shots as well. The model is clearly unfinished though as 343 Industries was testing out different weapon variations. How this ended up accessible from a Spartan Ops map and not the others is anybody's guess.
You can check out the video of the Forerunner Burst Pistol in the video gallery.
This isn't the first time that unused weapon models have been found in "Halo 4" as several were found after the Crimson Map Pack was released in December. To clarify, Xbox 360 owners who mod their console are able to access game files. You can't access this through normal use.