User blog comment:USMC Lance/Whats the better rifle: M16 or AK47?/@comment-4506250-20111014115433/@comment-356904-20111016083935

"The biggest issue in Vietnam was the ammo we used."

Which resulted in carbon deposits building up in the gas system because the powder used didn't combust as cleanly as the stuff the weapon was designed to use, and the grunts on the ground didn't have the tools to clean it out. That meant the gas system couldn't eject rounds properly, causing jams. The "grain" of the bullet (as in, its weight), had nothing to do with it.