I would like to know how to add a weapon like the repeater gun to a model.
(for example , the repeater gun to a skin)

What programs do I need?

Thanks

You need to follow the tutorial at massassi; the one about adding parts to models. It works the same way from the tutorial on, like, sp enemies.

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"The Answer to everything...Life, the Universe, and Everything, is...42!"
--Deep Thought
Hitchhiker's Guide to the Galaxy

If you are trying to make a skin into an enemy the easiest way to do it (often) is to put the skins onto an already existing enemy 3do.

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For the love of God, use the consistency checker!

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