By the way, Remington and Hastings make 20 inch - 12ga rifled barrels. My Hastings is a 20" barrel but they are hard to come by here in Canada. TradeEx is out of stock:
https://www.tradeexcanada.com/content/hastings-barrel-remington-870-12ga-3-20-scope-mount-slug--
For Remington cantilever barrels ... you can order them from Gravel Agency ... However, it might take 3 month for the barrel to arrive as it happened to me with one barrel.
Here is one that is 20 inches:
https://store.onlinecamp.com/en/replacement-parts-oem/remington/shotguns/model-870/3-barrel-assembly/f242441-bbl-12-20-fr-cant-n-s-sp-deer-matte-fully-rifled-bbl.html
And one more thing ... rifled slug barrels are very very ammo sensitive ... and each barrel is different. Be prepared to spend quite a bit of money on different sabots from different manufacturers ... and the time to test those ... and in order to find a combination that you can shoot accurately out to 150 meters.
Hope this helps ....
Faustus