Primers affect the pressure generated by the cartridge. Changing from standard to magnum primers may substantially raise the maximum average pressure of the cartridge and indiscriminate changes are not recommended. The A-Square Company conducted pressure tests involving six different primers. These tests used the 7mm Remington Magnum cartridge with a 160 grain Sierra BT bullet and 66.0 grains of H4831 powder and the results were reported in the A-Square reloading manual Any Shot You Want. A-Square used CCI 200 and 250, Federal 215, Remington 9 1/2M, and Winchester WLRM and WLR primers in these tests. They revealed a total spread in pressure of 12,800 psi from the mildest standard (the CCI 200) to the hottest magnum (WLRM) primer tested.
Changing brands but using the same type of primer will also usually result in pressure changes, but ordinarily these will be less drastic. In the A-Square tests the pressure spread between the CCI 200 and the hottest standard primer (the WLR) was 9600 psi. The spread between the mildest magnum primer (the Rem. 9 1/2M) and the hottest magnum primer (WLRM) was 8300 psi. These are significant pressure variations that cannot safely be disregarded.
Incidentally, these same tests revealed that the Federal 215 and CCI 250 large rifle magnum primers produced nearly identical pressures. The difference between these two primers was only 100 psi. A-Square also reported that, while they had not tested these two primers in all possible cartridges, this result was typical of their experience with these two primers.