Originally Posted by c3shooter
No, you don't.
Magnum primers have a pinch of powdered aluminum, burn hotter, giving higher pressures.
General rule of thumb is reduce starting load by 10%, but would check w/ manufacturer. Other folks may have advice for you.
the issue with magnum primers in very small charge weights is over pressure. there is so much energy coming out of a magnum primer its possible to get detonation instead of a controlled burn.
on the other hand since the charge weight in a 9mm is so small you probably wont notice it if it does.
if your getting flattened primers in a 9mm your pressure is too high.
cratering: where the primer metal is flowing into the firing pin channel
flattened primer. not necessarily sign of over pressurein some cartridges. but in a 9mm it is the primer has flattened and totally filled the primer cup.
none of these are ways to measure pressure. they are merely indicators you need to back off.