Another Nitro user here as well. Same Rhino .660 choke tube and the H378 3" load getting 316 inside a 10" circile at 40 yards!!!
I'm getting similar numbers from my 21" cantilever barreled, 11-87 and the Rhino .660 choke. The key to getting Nitro's to throw these tight patterns is a CLEAN barrel. I typically use Kroil and JB paste before each season as recommended by Nitro Ray himself.
After using the 3" Nitro for the past few years, and seeing it perform, I see no need for the 3 1/2" version, at all. Every turkey shot was DRT, including one shot at 62yds backing up another hunter using 3 1/2" Hevi-13. JMHO.