That would not be a good idea; like you said there are steel pellets in that shell. The hevishot pellets, even though the factory does not specifically label the kind of hevishot pellets they are, they will most likely be harder than steel. This is the reason there is a Hevishot line called Classic Doubles which are for older guns that cannot handle steel shot or for guns with tightly constricted fixed chokes. Your best bet is to buy an aftermarket choke tube capable of handeling non-toxics in the tighter constrictions. Before you buy a new choke tube, I would pattern your factory modified to see if you can get the kind of results you want before spending extra money.

I hunt waterfowl with a Mossberg factory full choke and it does just fine..