I was shooting 3" shells; the Remington #5 buffered loads put more pellets on the target than the Winchester #6 load. This is true even though the Remington shells had a quarter ounce less shot and larger pellets.
Once I had a super tight pattern, I noticed that it was way off center. I had to get a set of Truglo TG960 adjustable peep sights that clamp onto the rib.
I think Nitro is just one of the brand names they use, probably doesn't mean anything.