Where can I find a rifled barrel for my xtrema? Will any of Benellie's barrels fit the xtrema?
If I can't find or afford one by deer season can I shoot rifled slugs in my optima plus barrel and if so what choke should I use?

yes you can shoot slugs through your gun. I would use an Improved Cylinder choke if you have it. I doubt Benelli shotgun barrels will fit a Beretta shotgun.

go out to shotgunworld.com and see if you can snag a used one. No you cannot use a Benelli or any other type barrel on an Extrema. I would use a wide open choke if you plan on shooting it in that barrel..it is backbored as well