We need the photos Snapper, but FWIW, a goodly proportion of 12 bore rifles had fairy fast twist, and didn't use Huge charges.
Most soft -skinned dangerous game rifles were usually set up for ball, as it was a better stopper than a conical.
The favourite rifle of Agnes Herbert was a 12 -bore, throwing 5 1/2 drams (150 grs) of black and a round ball.
Re the crack, sometimes we see a crack glued and also a thin layer of linen glued inside the channel over the damage,( If it will fit!)
Good luck getting it sorted!
R.