Percentage to back off using Magnum primers

[Boriqua]

I have used magnum primers for 38 and 40 but never with plated bullets. I always backed it off from the max load and my 38's were usually 4.0 gr of HP 38 using a magnum primer. Gave it a nice pop.

I am loading up some 40 in plated bullets. I dont have a ton of experience with plated so I checked one round and realized my crimp was to heavy because the plated bullets dent so easily and backed it out and I loaded 165gr plated with 7.3gr of longshot and standard small primers. Nice little pop and certainly warmer than the factory white box I brought. It was also very accurate but there was some flash.

Here is my question .. I remember at some point hearing on one of the local boards to back out ???% when using magnum primers. I thought it was 10% but I cant swear so I am looking for some confirmation. I have A LOT more magnum primers than standard so would like to use them in a batch I am going to roll tomorrow.

Hogdon lists max for 165gr plated bulleted at 7.8 and min at 6.9. I figured I would be safe at 7 grains using a magnum primer??

Is 10% real or something that just got stuck in my head?? Is it a good general rule??

 

[Old Jeff H]

From what I've read, it's safe if you start at the minimum load and work you way up (if you want to work your way up).

In 40 I've ran as high as 8.0gr of Longshot with 180gr JHP (settled on 7.5 because I wasn't getting much of a gain past that) and standard small pistol. I would think you'd be more than safe with 7.0, plated 165's, and magnum primers. Do you have a chrono to run a standard/magnum comparison?

 

[Flash]

Boriqua,

Check the video of Super Vel testing small pistol, small pistol magnum and small rifles with all other factors remaining the same that are on this board and you'll have your answer.

https://www.arizonashooting.org/forum/viewtopic.php?f=29&t=11435

 

[samnev]

I backed off 10% of my 9mm loads which were relatively mild to start with just in case.

 

[Flash]

I used magnum primers with 9MM loads once and .45 ACP once and just used my regular load but then I use middle of the road loads most of the time anyway.