I picked up a box of remington small 5 1/2 primers because I thought they were regular small primers. The box isn't marked magnum. I found out they are mag primers. Would you shoot these in 38 Sp or 9mm loads and reduce the load by 5 to 10%? My current loads should be about 3/4 mark so I'm not at max.

reduce load by 10% + they will work fine - good articule in this months shooting illustrated on reloading for economy -

I have never had a problem. The biggest difference between pistol magnum and non-magnum primers is in the strength of the cup (i.e. so it is not pierced or blown out by heavier magnum loads). 10% less powder in the load with a magnum primer is a good starting point. The only powder I have ever read of that doesn't like magnum primers is AA#9. The people who were getting overpressure signs with magnum primers were 10mm experimenters trying to squeeze max velocity (i.e. 1300fps+) out of their loads. If you are not doing ultra-hot loads, it shouldn't be a concern.