I've had issues with NS... But man come on, be reasonable.
You CANNOT expect a warranty centre to let you do repairs that you are nowhere near qualified to undertake. Would you replace a TV backlight yourself? No. For guns it's even more crucial the repair is done right, for OBVIOUS reasons. To make matters worse NS could open themselves up to significant liability by doing things like that. Not a good idea in any case for anyone involved.
You also can't expect someone to send you a refund check without giving up possession of your goods. That is absurd.