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If a chaos marine takes the +2 close combat weapon option, does that mean they can fire bolters and switch to the cow and bolt pistol? Do they have to choose at the beginning of the turn which weapon(s) they are using, or is it possible to rapid fire and switch, prepping for an assault? I haven't been able to figure this one out, and I'd be very grateful for some clarification.

The weapon you fire in your shooting phase/overwatch has no bearing on what weapons you use in the assault phase(s).

but if you shoot the bolter you cannot charge.

If I rapid fire I can't charge, but I can use the ccw and bolt pistol for a retaliation assault?

Yes. As I said, the weapon you use when shooting has no bearing on the weapon you use in assault.

Addendum: With some exceptions like burnaz.

Awesome. Thank you guys for clearing that up.

A further clarification - neither the Shooting phase nor your Assault weapons dictate what weapons you use for Overwatch.
So if you're charged, you can choose either the bolter or bolt pistol.

Better clarification: if you want to charge, use the bolt pistol; being charged, use the bolter. Other than that, yes, the 2 points per model buys an extra attack.