The UAC was manufacturing weapons, and one guy thought "Hey, why not make a gun that fires MULTIPLE shots?" After he pitched it, the HS was mass produced, and it was his best career choice.

This is pretty easy, the gun seeks ownership and to do that it puts itself in easy spots. You should find the HS one a box somewhere. Beware, as unholy beings will try to claim the gun for itself. Use it to send them back to where they belong.
In Delta Labs 4, after going to Hell, the HS won't appear in your inventory. Do not panic! The gun is found later as your arsenal got fricked into different areas.
When exiting demon land, you may lose it again. It is found later again.

The gun is holy due to it's power. Cacodemon, sent back in minimum of 2 shots, proving it's holy power. It can destroy most who oppose you, and is cooperative with the Soul Cube, so use it to easily rack up the souls.