Originally Posted By: erslllPMAGs are the exception to this rule though. I haven't had any luck getting more than 3 or 4 .204 rounds in a PMAG.
+1 The .204 Ruger is longer than the .223/5.56 and can have feeding issues because of it. Early on, many builders struggled with feed problems with the .204. A modified chamber-mouth and good magazines will solve most of the issues. I have tried P-Mags, and where I love them for the .223/5.56, they didn't work well for the .204 when I tried them.
The geometry of how the magazine is held in the lower receiver has much to do with feed and function. You can take an upper receiver that works perfectly with a certain lower receiver and magazine, move that upper and magazine over to another lower receiver and have nothing but problems.
The mil-spec 5.56 surplus magazines have worked as well as any I have tried.