From Regular to Magnum Primers, how far to back off on powder?

[cbh560]

I've got a few loads worked up for my .223 using H335 and CCI 400 primers. I've been doing some reading lately and lots of guys are saying that the H335 really shines when using magnum primers so I thought I'd try them out. How much should I back off on powder when I switch to mag primers? .5gr and work up? 1.0 grain and work up?

This rifle seems to really like most bullets with the H335. I've got good loads made up for 40gr VMAX, 53gr VMAX and 69gr Sierra MK's. Should I back off on powder a different amount for each bullet weight?

It should be noted that I was able to achieve .3 to .5 MOA loads using standard primers, however, due to a lack of availability and suggestions from many gunnutz, I bought magnums (CCI 450's) to give a try....2000 of them, just to stock up a little while I was at least able to find those in stock.

Thanks

 

[Ganderite]

I changed over to 450s because I had to. i was getting some ignition failures with 400s. I have also started to use the Lee factory crimp die, and it has helped ignition. i know that because I ran a couple thousand rounds already loaded with the 400 and it has almost cured the ignition problem.

To answer your question, I would drop 0.5 and work up, looking for the accuracy you had before.

 

[trevj]

How far to back off? Gonna depend on what powder, how much, how close to a max load it is in your rifle, etc.

Got a chronograph? It'd be of use.

I'd not flinch just sticking them in, and going to the range, with MY loads, but mine are staid, low to middle of the range loads with lots of room to increase the powder charge or otherwise increase the pressure.

Cheers
Trev

 

[Groverino]

I do .5 gr but I don't load anywhere near max.