Beretta 686 in 12ga, and add a set of Kolar tubes which will also allow 20, 28 or .410 to be shot in the same gun (depending on which tubes you have in at the time, obviously). You can probably get just a set of 20ga tubes, if that's all you want. That will still give you a very versatile setup. The tubes just slide in with no tools required. They can also be had with changeable chokes. Very nice system which works very well. I think a single gauge tube set retails for about $550-600.

Yes, costs more than an 870, but still within reach of the average shooting enthusiast.
A great all-around gun, very solid and well built.

That would be my choice.

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