Free: Contests & Raffles.
I would open it up to an Improved Cylinder or a rifled slug choke.If you have screw-in chokes you can change them to whatever you want, as long as they are designed to fit your particular model of shotgun.
That gun appears to have a Poly-choke. See this for more info: http://www.poly-choke.com/InstructionPage.htm
The short answer is yes. if all you have is a modified (screw in or not) you can shoot slugs out of it. ( not sabot type slugs, but brenneke or foster lead slugs)- yes this includes the plain Win, Rem or Fed rifled slugs) Do not use slugs in anything tighter than "full" (like the ex-full turkey tubes). You might get better results with an improved cylinder tube, as it has less restriction- or you might not.
sniper you are entitled to your wrong opinion. I did the research about what you could shoot out of what choke when I first started shooting shotguns. I have since fired slugs out of probably a dozen different shotguns with fixed modified or full chokes, and screw in chokes in mod or full. I'm still here and have never damaged a gun in doing so.. the cut and paste below is from Mossberg's customer service "frequently asked questions " page..Can I shoot slugs or buckshot through a "smoothbore" field barrel with Accu-Choke™ tubes? (500/505/535/9200/88/930 models) -Yes, and with sabots or rifled slugs, the more "open" the choke tube, the better the accuracy ("Improved Cylinder" recommended.) The "full" or "modified" choke tubes are preferred for buckshot loads. Slugs, buckshot and steel shot loads are not recommended for use with an extra full "turkey tube" installed, due to the tube's tight constriction. Do not fire any load without a choke tube installed, doing so will damage the internal choke tube threads.
no hard feelin's Devil Dog. If you err on the side of safety based on what you have seen- no one can fault you for that. I do what I do because the manufacturer said it is fine, and it's what I have always done. My first gun was a shotgun with a fixed full choke- so it goes back a ways.