A few years back, my late wife got me a Farquarson Rook rifle that originally was in 300 Sherwood. Some idiot bored to .410 shotgun to avoid having it on his firearm certificate. To add insult to injury the actual chambering was not inline with the bore either. Please understand that this rifle is a jewel made by a famous English firm, with lots of engraving and wood that looks like the finest Italian Marble. 

Rather than experiment on it, I used a POS Steyr made version of the French Gras that was literally a rusty pole. The original chambering was in 11.5 French.

I drilled the barrel with a half inch piloted drill bit and counter bored the chamber to 15mm. I also counter bored the rim to 17mm thus cleaning out any trace of the pitting there was present.   I turned a plug to fit the above dimension and bored it interference fit with the half inch .357 liner I was using for this project