For a LONG time now, T-C has made .45 Colt/.410 barrels that accomodate 2-1/2" or 3" .410 shells.
They have a special "Choke Tube" that's made to STOP the spin imparted by the rifling.
You don't want to shoot the .45s in there with the tube "in"!
There are 2 versions of this barrel.
One with the good "pistol" sights the Contenders normally come with.
And the other...the one with the big (shotgun) bead on the front and a rudimentary "V" notch on the rear, all on a wide Vent Rib (gotta keep the "intent" in place here

).
You can pretty much tell which barrel's for which.
They HAVE to do this because smoothbore pistols aren't legal (since 1934).