The .243 isn't unique and your high primers have nothinh to do with high pressure, in fact it's the reverse. All primers back out as far as the headspace allows. At normal pressures the bolt face will reseat them as the case stretches back to fit the chamber. Your proud primers are telling you that your pressures are much too LOW or they would be flush with the bolt face!
Primers standing proud by 12 thou says you're setting your shoulders back at least that far and that's far too much! You're setting yourself up for some nasty head seperations ... soon. Back your sizer UP an eighth turn plus a bit more and check to see if the empties chamber properly - refine the case to chamber fit from there with SMALL die changes.
And move your powder charges up too, you're not even getting complete combustion now.
I don't know if a higher grade of scope or better sling swivels would help hold your primers in with really low pressure loads or not. ??