With 303 British the thing to check with brass cases is the measurement of the case at the web. Most American made brass is undersize at this important point, a useful thing to some as we use these cases to make 6.5x53R cases, if the web is the correct size the cases will last longer as they don't have to stretch at this point where the case wall thins to fill the chamber. If it is undersize as most seem to be then the case has to bulge out to fill the chamber and lower case life is the result. They should measure 0.455" at the web but most are only 0.450" and if the chamber is slightly loose as a lot of ex-service 303's are then the cases have to expand even more which weakens them. I personally use a lot of HXP 69 brass and have not had a failure with them as yet. It was made to the correct specs which helps i am sure. Good luck with your handloads.