did you file fit your rings, or did the machinist? aside from running all forged internals, did you tell him it would see nitrous? NA motors and boost/nitrous motors require different ring gaps. when in doubt...wider is better. the only thing that's bad with wider gaps is more blow-by, but if you have a properly working PCV system, it'll suck those fumes out before they can cause any real problems (if it doesn't work, you'll pressurize your crankcase and create oil leaks, and contaminate your oil with exhaust faster...so keep it working!)
the benefits to slightly wider gaps FAR outweigh the negatives in my opinion. if your gaps were too tight for nitrous, the ends could have butted together causing the gouges you see, and it could also sieze the motor.
My money is still on burnt or broken rings from the NO2 shot. that, or...when starting the engine the first time, did you ahve any fuel issues? running too rich or not starting? another possibility is that you washed the cylinders down with too much fuel--reducing the oil on the walls which could also prematurely wear out the rings and cause scoring.
