I accidentally bought magnum primers the other year and put them in my 9mm cases. Would I still be able to ise them or should I just knock the primers out and start over? I never loaded any of them since I realized it right afterwards. I am going to be using 90 gr. Hornady gmx and longshot powder.

If you don't want to buy new ones, or trade these, I would start with starting loads. Should be fine.

quote:

Originally posted by dpcd:
If you don't want to buy new ones, or trade these, I would start with starting loads. Should be fine.

+1

Reduce your normal load by 1/2 grain,you'll be fine.

I accidentally load some 40S&W with magnum primers and didn't realize it until they were all loaded. I don't load pistol cartridges at max and when I fired them the recoil was a little stout but they shot fine. A funny story since we are talking primers. I helped my brother load some ammo for his 300WSM and I used my go-to load for this cartridge except forgot to use magnum Win primers and used Win standard large rifle primers. He tested them after he got home and they shot into a group you could hide behind a quarter at 100 yds., but he said the recoil was heavier. He thought we loaded them hotter than previously but the primers were the only change. Who knows!

The effect of the hotter primer would be to start more of the powder burning on ignition- basically speeding up the burn rate of the powder, which "all else being equal" would run the pressure up. so as noted, if you are at the starting loads and have a margin for an increase in pressure, it should be fine.

Many of us who load 9mm for open bolt subguns use small rifle primers to mitigate against perfs and OOBs. Start below what works with SP primers, then run them over a chrono. Half grain usually is about right, same as for magnum SPs.

The safe answer is as above, drop a half grain or so (10%) on the load and work up.

Having said that, I really didn't see much difference in SP, SPM, and SR primers in either 9x19 or .40, myself. I used whatever I happened to have available, but as they were competition (IPSC) loads, I did check them with a chrono to make sure there were no issues. Maybe 20-50 FPS difference. The magnum primers did seem to be less accurate as a whole than the standard, and the rifle maybe a bit better.