For what it's worth My shotgun which I refer to as a Mossberg model-500 is actually a Newhaven Model 600, I bought it used in March or April 1984 for $109. I keep using it though I've since bought two Remington 870's, one of which I dumped a ton of money into, and now I'm belatedly putting money into it I'm looking to put a cantilever Fully rifled slug barrel on it to make it the equal of my 870 Express that I lavished so much attention on in the late 80's (Hastings rifled barrel, Trap-style butt stock and an Aimpoint), I have several Spare Aimpoint scopes and as a fast working tool for killing deer you will be hard pressed to build anything more effective than a Red-dot sighed 12Ga Sabot slug thrower
But my New Haven 600AT came with a plain, non-ribbed 30" full choke barrel, it does have dual action bars.