Sean;
Check the brass from the once fired factory ammo to see if an insipient seperation has already occurred. This brass like most of the belted cases is made a bit 'generous'. But brass will stretch a long way before failing. If no crack or depression is found inside the case just above the belt then the brass, sized properly, will live for many reloadings. I didn't check the chamber drawings but all the short mags are very similar. I'm not sure a 338 would help. You would probably have to go to a longer case, 300 Weatherby, 300 H&H, etc. to get a case that could be formed to a case that would be ideal for your individual chamber. Unless the stretching is gross I believe this would be overreacting. This is a known problem in all the belted magnum cases and has been lived with by others for many years. It does reduce case life some but most of us should most likely get new cases more often anyway.. although I have some 222 Rem brass from the 70's that's been reloaded more times than I will admit to. You are really dealing with 2 sets of tollerances here- the rifles and the cases. If the cases are safe (check for the ring on the inside) just reload and enjoy, discarding when the necks crack... I have a 300 Win and a 7mm Rmg mag that have the same problem. I have cracks forming in these cases after only 6-8 reloadings.. Although measurements indicate extreme stretch when firing factory ammo.. good handloading has made case life acceptable.