You evidently have a modified choke on your particular gun. The manual I saw for that model indicated that a modified choke is not recommended for slugs. That model also seems to be offered in more open chokes that they say are suitable for slugs. It is not the model of gun but rather the choke on your particular gun that they say should not be used to shoot slugs.
I am curious about their notation of the system for that gun being gas. A gas operated pump shotgun is something I have not seen.
