I have an older (80's)Mossberg 500 12 gauge pump shotgun w/a 28"26?) external choke smooth bore barrel, which I believe came stock. I have recently purchase an assortment of barrels to swap onto the gun including:rifled slug, field, & turkey. The problem is that none of these fit the gun!

I assumed that all barrels would be interchangeable but apparently I was wrong. Does anyone have similar problems or know the history of this shotgun?

PS- I will gather the model information soon.