I had some 22 LR chamber inserts made from an old 22 barrel for an antique 30 rim fire revolver... with less than a 1/2" of rifling left, in the inserts after the chambers were put in, it shoots suprisingly accurately, & the 22 caliber bullets don't touch the 30 caliber barrel ( much smaller than a 12 ga )
... so I see no issues shooting the 22 in a 12 ga... other than sighting, & loss of velocity ( if the rifled portion were no longer than a 12 ga shell... of course if it's for a break action, & had a longer barrel, the velocity would be comeasurant with the barrel length )... & with the cartridge being as cheap to practice with as a 22, there is no reason ( since I'd expect it will shoot to a consistant point of impact ) that you couldn't learn where to place the bead for impact at your target