Well, it doesn't have to look like the gun you'd use. The pulse rifle mod looks nothing like a Thompson, yet holds a Thompson as it's core. That was just an example. For example, you could have something like a P90 at the core and an outer body built over it to make it some wierd looking prototype sniper rifle, and you'd never know it was built from a P90.
But I understand what you're trying to get at. You want to create something totally differently and unique. Just suggesting that it may be easier to attain that goal by buiding onto something else rather than going from scratch considering the potential legalities of it.
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