Interesting...since I went to Gunbroker, did a
search for slug barrels (using the term "slug", since most slug barrels are assumed to be smoothbore), and the result came back with 151 hits, about 2/3rds smoothbore--or at least, no mention of rifling (and a quick analyisis of several of those not mentioning rifiling, none had it in the more complete description).
Considering how long smoothbore slug barrels have been around, and how recent of a developement the rifled shotgun bore is (relatively speaking)...I guarantee you there are a heck of a lot more smoothbore than rifled out there.