During the recent reloading component shortage, I've known experienced reloaders who substituted magnums for small pistol primers in their reloading processes. Those Few Folks always used the lowest published powder grain loads in the handloading book when they substituted the magnum primers.
I watched some shooting from these reloads. From what I saw as the outcome, there was no discernible evidence of high pressure. No cratered primers, no flattened primers, no hard recoil, no Big Boom......nuthin'. And not very much (1-2%) increase in chronographed velocity. So--From what I've seen, you can't tell much--if any--difference.
Yep---I used magnum primers in my 9mm reloads for a couple-of-thousand worth of ammo. Dropped the powder charge a bit, chronoed, brought the powder charge back up, chronoed...
...found no effective difference when I did so. Note, however: I'm using this in 9mm at medium velocities, so I'm not reloading a high-pressure round in the first place, like .40.
Next time, if I need to use magnum primers again, AND am still using the same powder/reload recipe, I'll won't even change things at all. (If I change powders, I'll check it again, but don't expect issues.)
...but like I said, that was 9mm, not .40. Don't really expect much difference with .40, but I
would back the charge down to check it out first.
