One short question really:

Let's say I'm Assault Marine and I recieve chainsword as starting wargear.

In case I would like to get an exceptional chainsword for this mission how many requisition points I need to use?

You have to buy a whole new chainsword from the ground up. You generally can't just apply a quality modifier to an existing weapon; it has to be built on that quality (unlike, say, adding a sight to it).

So the cost of taking a new, better-quality chainsword would be (base chainsword cost x quality modifier).

And then you have two chainswords! Unless you choose to leave your free one at home, I guess.

Thank you very much for clarification!

You have to buy a whole new chainsword from the ground up. You generally can't just apply a quality modifier to an existing weapon; it has to be built on that quality (unlike, say, adding a sight to it).

So the cost of taking a new, better-quality chainsword would be (base chainsword cost x quality modifier).

And then you have two chainswords! Unless you choose to leave your free one at home, I guess.

I never leave my chainswords at home.