I've never really noticed primers making such a huge difference as you mention 303.
I have two bolt faces scarred by winchester primers which put me off them for everything but bp and cast bullet loads.
Curgently using the last of a batch of murom alongside cci std rife primers. It may make a difference with magnum cases and ball powder or if you are target shooter but I don't notice it.
At present getting any primers is a bit like winning the lottery
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