From 1941 Winchester had big quality problems with 303 manufacture, It got so bad that the ammunition was banned from use in aircraft and some batches were so poor that they were not even issued to active Infantry, and instead, consigned to 'practice use only'.
The problem was a lack of 'dimensional' quality control resulting in jamming.
No serious problem if you are just 'at the range' but when being shot at it could be quite inportant to be able to fire back.