Historic bore. Double rook is 297/250 confirmed by Mr Winks ex holland & holland. Not stamped on gun. Uses same round as my single shot rook, it is stamped 250 semi smooth bore. Watson577 rifle next to rook is 375 2 1/2 inch royal with a extra set of 28 bore paradox barrels with a seperate fore-end. I have managed to get boss&co to build a 375fl sxs double with extre set of 303 barrels using same foreend it was a lot of work to contour the barrels to suit 1 for-end instead of simply supplying a fore-end for each barrel.