I got some ideas for new weapons:
1) Radiation Riffle
This one should fire massive wide-range RTG rays. The effect would not be immediate, but the bigger "life to death ratio" is greatly compensated by the horrible deaths of ugly aliens. They would scream in pain and begging for quick, merciful shot

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2) Virus/Poison spreader
The effect is the same as the one mentioned above (slow horrible death), but this one adds new strategy feature: the infected person/alien spread the disease, co it's extremely helpful in situation with hordes of enemies and plenty of time. Use with caution, Duke has to be vaccinated

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3) Squisher
Equipment capable of space distortion. It basically shrinks the space around the target, so it gets squished by walls or anything nearby.
4) Suicider
Ray gun causing suicidal behaviour. The inflicted alien can also drain the mood of other aliens (which leads to worsening of their skills)
5) The Apocalyptor
The Ultimate weapon. No one knows why, no one knows how, but it destroys everything in one big explosion (maybe nuclear explosion?)