Hogdogs, I tremble with fear at the thought of contradicting you about anything shotgun, but as far as I know, Mossberg makes 18.5" barrels for the 870 in 12 gauge only, not in 20 gauge. Remington makes an 18.5" rifled barrel with a cantilever scope mount for an MSRP of (I think) $240 -- ouch. Otherwise -- I've been looking for a while, and as far as I can tell, there just isn't one made, without rifling and vast expense, for the 870 in 20 gauge...
When I get around to it, I'll try to find a used barrel at a good price, get it cut down, maybe have sights put on it -- that seems to be the only solution to this one...