I'm looking to work up a 135 gr. hollow point load for my 10MM Auto. All load data I can find calls for a Large Pistol primer. My question: What would be the effect on pressure by substituting a Large Pistol Magnum primer?? I've got a scad of them and would like to use them if possible.

I seriously doubt that you'd notice the difference.

If you are starting to work-up properly, ie., starting light and working-up gradually, it will make no difference. HOWEVER...if you have already selected a load using one primer and then switching to a magnum primer, it is not a wise thing to do.