Hey everyone--
From what I have read, small pistol primers have a slightly thinner cup than small rifle primers, meaning they can't be safely used at rifle pressures (you'll get pierced primers and or high-pressure failures).
However, I don't have small rifle primers right now and would like to work up a few low pressure test loads using a Lyman 225415 55 grain boolit for my AR-15, to see if I can find the sweet spot for accuracy, reliable cycling, and no leading. These CCI 500 small pistol primers are what I've been using in 9mm since I first started casting. My favorite load with Bullseye in 9mm gets me to an estimated 28,050 CUP and I've never once had an issue with primers blowing out or anything like that.
The highest pressure load for my test rounds here is 14 grains of IMR4227, which should only generate 23,200 CUP of pressure. I have even read that some precision .223 shooters use small pistol primers in their loads specifically because it improves accuracy, although that's anecdotal evidence at best.
Am I safe to use small pistol primers in this situation?