Primers are as much a variable component as any other in reloading and can alter velocity and trajectory and therefore accuracy quite considerably.
I have long been an advocate of using primers in the same way as powder to vary a load with great effect.
We dont choose our case because one looks shinier than another but because of the chemical composition that is most suitable for our particular load. Neither do we choose a Powder just because the local shop has it on the shelf and so with Primers - choose your primer to do the best job possible for the load you are generating.
A change of primer can in some loads require a drop of 3 grains and the difference in velocity without any increase in pressure can equally improve by 100fps. Use your primers in the same way as your powder and you will be surprised at the difference your loads will provide.
Here is a basic heat chart - from top to bottom drop about 5-6% in powder and work from there.
http://img.photobucket.com/albums/v356/mry716/Shooting - other/primerheatchart.jpg
Use it as a guide and add in your own primers when you have assessed them. eg, the Prvi Partizan Primers although burning approximately as hot as the Winchester range do because of their different construction not seem to require a reduction in powder when changing from say Remington. Try for yourself and let us all know your test results as it will expand the above chart considerably.
Use of Magnum primers is normally reserved for the Ball Powders but I am finding I can use them to great effect in the shorter stumpier cases such as the WSM and WSSM range of cartridges. Logically this should also be valid for the Benchrest PPC cartridges.
The same is true for the H4831 SC powder in 243 - A Federal Magnum primer seems to provide anything up to 120 fps higher velocities than standard primers yet seemingly without additional pressure.
BUT
do drop the powder weight and build up again.
In a recent 'Handloader' I notice that Charles Petty has written an article on Primers and 223 accuracy - anyone who would like to read it email me at
mry716@hotmail.com and I will send a copy.
Please remember to drop powder weights when changing primers and do also remember that with the present hot weather you need to reconsider all those max loads and take out a little powder.
A 10 degree rise in temp requires a 5% decrease in powder. If we are going to get such varied weather in coming years it may be best to have a min, intermediate and max load to use dependant on weather temperature.