I have a substantial quantity of large pistol magnum primers which I rarely get to use.I'd like to run them off in my .45 ACP, for which I reload much more frequently. I was thinking of a half-grain reduction in the powder charge to compensate for the magnum primer's longer or hotter flame. Anyone ever use magnum primers in non-magnum loads?

i doubt that you have any problems even if you keep the load the same

Unless I was sitting at the ragged edge of the max load I would switch primers and go.