Reopening this old thread. With Primers so scarce picked up CCI 550 SPM. Usual Titegroup 40S&W loads are 4.2 or 4.5 grain. Going to start below 4.0 and work up to 4.2 and see how it goes.
No problem using magnum primers in your 40 S&W! I have used magnum primers in my 9mm as well. If that is all you have, use them up. Just avoid loading maximum pressure. I always go halfway between starting loads and maximum loads when loading a particular caliber. I have even used Large Pistol primers in my 308 rifle loads as there were no large rifle primers to be found. They worked perfectly.