[broom_jm]

There have been many studies of varying scientific worth on the difference between using standard and magnum primers. The only undeniable truth I have seen/read is that you should definitely back off the powder charge and work up your load again. You may find standard primers give greater velocity than a magnum primer and you may find that it makes no difference, whatsoever. The only safe thing to do is follow good reloading practices and work up your load from scratch.