How did soldiers manage to get replacement weapons, like a 1911 or AR-15, when their M16s malfunctioned in Vietnam?

I’m definitely qualified for this answer. During my year in Vietnam, I spent my last 6 months as the acting Supply Sgt. I stayed in the field as part of the command post group. That means I usually got to sleep on the top of the hill at night. Before going out on an operation, I was issued an M16 and a Colt 45. The first Marine having a problem with their weapon in the field was given my M16 and I took control of the non-functional weapon. The next time we received supplies, the non-functional weapon was taken to the rear by any personnel that was leaving the field. I would then usually get their weapon right before they boarded the chopper. There were enough of us in the command group that had extra weapons. We would keep the Colt 45’s and hand off our M16’s to any Grunt that needed one. Mine was always the first to be given up. I had just given mine up in this photo as we were moving out. And yeah, that pack was a killer.