Look into the H&R slug guns. They definately won't bite too badly into the budget.
www.hr1871.com
im with Friz, NEF slug (rifled, iron sights) about 125.00 at walmart, or the H&R Ultra slug hunter (heavy barrel, cantalever scope mount, swivels, rifled) for around 200.00
i bought the ultra slug .20 ga this spring. nice grouping with lightfield slugs, fun to shoot