If the shooting was on the numpad arrow keys as well, then it would not make my arms hurt. Closed it when it started to hurt.
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I specifically recommend the numpad, because those keys are closer to the mouse on a normal keyboard (and people normally use the mouse for shooting like this on a pc). The current controls work well on a laptop without a numpad. If you'd go further right, then you'd hit keys that surely change between different languages, and more annoying than the swapped y/z on some keyboards.