They could be Rigby loaded in which case the bullets will be steel jacketed. RWS also loaded with steel jacketed bullets which some mistake for cupro-nickel (same silver colour) but their case heads were stamped RWS. Use a magnet to confirm if steel or cupro-nickel. The latter often riveted or broke up if hitting bone (apparently) so would not be as valuable as cartridges loaded with steel jacketed bullets.
From my experience with earlier factory ammunition, Kynoch at least, is that a percentage of the cases suffer neck splits on firing. If lucky enough that they don't, then anneal before reloading.