Anything funny or serious. It doesn't matter. I just want to get a discussion going. I would like to see something like the arc cannon or "gravity cannon" from District 9. Another idea I had is called the "Time Rifle." When you shoot it at an enemy, he's forced into a slo-motion state. It's sorta like the slow-time shout in Skyrim except only the target is slowed down. You can either mow em down with a gun and he dies in slow-motion or you go in for the kill and do an awesome hand to hand finisher.
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I'm not sure about in a rifle form, but I think a grenade that significantly slows down time in an area, including projectiles and grenades, would be pretty rad. You could obviously use it offensively to paralyze enemies before going for the kill, but you could use it defensively too, creating temporary cover or blocking a doorway. I approve of that big time.