Dear Ovidio,
This is a frequently-discussed problem, you'll see many threads and diagrams about what happens. Here's my personal experience:
I had a MLE which was in lovely condition, but it split brass and in the end I just sold it on. I have never subsequently had any problem with any of my other .303s, so I think it must have just had an oversize chamber. Short of getting rid of it, here are simple ways to reduce the tendency to split brass:
1. use good quality cases - avoid Sellier & Bellot; choose Prvi Partizan or Greek surplus HXP.
2. Segregate your cases, and neck resize the ones used in the problem rifle [don't full-length resize].
3. Ask your gunsmith to check the headspace, and change boltheads as needed - though this is not likely to be a big contributor to case splitting. When you say you have a 0 bolthead on a No 1 rifle, this puzzles me somewhat, as I understood the No 1 boltheads were not numbered for size.
all the best
Rob
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