Today I tried some Speer plastic training rounds in .38 Special. I was using Speer large magnum pistol primers #350. Using my S&W 642, my problem is that only four of the 25 rounds I loaded fired. They were all loaded correctly and all had the primers indented, I tried multiple times but these primers will not fire. I tried some Winchester large magnum pistol primers with the same result. This revolver works perfectly with all types of live ammo. Is there a problem using magnum primers or is there something else going on? I just fired five rounds of live ammo with no problems. Thanks for any input.