Your load is likely producing too high of pressures to use the softer SP primers.
FWIW, there was a reduced power/noise .223 "gopher load" being shared on a varmint hunters forum some years back that advocated use of SP primers (and blue dot shotgun powder.) As I recall, there were a half dozen guys building and shooting them. Their goal was to get Hornet-Bee performance with low sound signature. They worked exactly as intended! I built hundreds of them over the year and found the rounds to be very accurate in every .223 I've owned (barrel twist permitting for the 45gr bullet). The small pistol primers worked flawlessly in this application as well. My thoughts are that one test in one rifle should not be the basis to dismiss an the potential for an entire concept. We learn and move on.

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Dave