I have had the same case separations in a Bren of mine. But mine was from PPU brass and Wolff steel cased ammo, so commercial ammo. It improved and ran well when I used MEN .303, but that is not so easy to come by any more. HXP would be a good alternative. Don't bother with POF, unless someone here has a different opinion. It is getting to be old ammo and my understanding is about 50% of the time, the POF (Indian manufactured) won't ignite, so you are constantly dealing with hang fires.

If you can find someone with .303 head space gauges (No-Go and Field), have them look at your headspace. It is pretty easy to do.