Sounds like a fine pattern to me.

Couple questions. How long is your barrel? A longer barrel will improve the pattern.
How many shots did you take with the Nitros? I ask cause I've heard guys talking about taking only one or two shots and being done.
Were you using a Lead Sled or sand bags on a bench? When sighting in or patterning my gun I like to make sure that I am as steady as possible. You need to take all variables out of the equation. Did I flinch? Was I steady?

One more thing....As much as I love the Nitros and how they pattern at long ranges....try being patient and make shorter shots.