Primary fire is the classic mode, but not only can it shoot through multiple people without slowing down, it can also shoot through walls. Fire a black hole where ever you want to go, and then shoot another one at your feet. Activate the shield and jump in to teleport to where ever you fired. That's really not a useful feature, since you have various forms of weapon-jumping, not to mention the Translocator. This replacement for classic U4E's QuadShotgun features one of the best models in the entire pack.
Primary fire shoots one barrel, and alternate fire shoots both barrels.
It'll fire both chambers, even though one is supposed to be empty. If you played U4E from the beginning, you'll recognize this weapon, and probably have missed it in its absence. But this weapon's namesake is its alternate fire mode. Hit someone, and they shrink to a very small size, and will explode from any impact. If they're not hit within a minute, they'll grow back to normal size.