Doug,

It's stating that the Mossberg choke may constrict the shot too much. This is a simple disclaimer stated by most companies. I purchased a new Ithaca last year, and their own X-full turkey choke is not rated for Hevi-shot. However, after contacting Ithaca tech support, they actually pattern the gun with #5 Hevi-shot. But you still load & shoot at your own risk...

You could easily however, if it bothers your conscience that badly, purchase an aftermarket choke (Rhino, Jellyhead), which are manufactured to shoot the denser shotloads.

S&R