I swear I've seen this asked here, search foo is weak this evening. I have already removed both pins, separated receivers, then gently pried the bolt loose. Problem corrected. My question is how do I stop it from happening again?

Its a newer bushmaster xm15 factory gun. The cartridge I removed was dirty and greasy, but fits the chamber fine. It was a 55 grain soft point and the point was not mashed or sheared. Am I getting too much lube in the chamber when I clean? I have had only one other malfunction with this rifle, I blew out a primer and it got into the feed ramps.

On another note. Before this it made my first sub moa five shot group with barnes 53 grain xls.