9,3x74R brass need a little work on the lathe for turning down the base to 303 diameter but I dont feel comfortable with its rim thickness. the 303/375 flanged have a 1,6mm and the 9,3x74R have a 1,4mm rim thickness and thats a little bit to much if you remember that the max. head space tolerance should be not more than 0,15mm.
I would not hesitate to drill the kynoch/bertram brass flash hole's. they are maybe not regular yet and it will be only better in the end.
hornady 405 win is the best brass you can get now, it need a little work on the lathe but you do it only once. its no problem to turn 0,1 -0,5mm from the base and the case will chamber. I do this with several cases: 9x57 mauser, 11,2x60 schueler, also 24 ga Magtech brass because it dont fit into my old begian hammer gun with its very tight chamber.