IMR 4227 is a fast powder. My guess is your port pressure is low and not enough gas pressure to work the action (doesn't mean chamber pressure is low). Try shooting just one round with an empty magazine in the rifle. See if the bolt locks back. It very well may be that the combination of the light bullet for caliber and fast powder is not compatable with the gas system.

Edited by Siringo (02/16/16 02:45 PM)