Go to a gunsmith with your gun before you try to purchase any barrel. From what you describe you have an older model 870. I am not so sure the newer model 870 barrels are that adaptable as Remington cheapened up theur guns with some stamped metal parts.

And others are correct - it might be able to have screw in chokes added to that - I am going to a gunsmith internet bud I know and will ask him if it can be done - how much it will cost you.
JW