Not only is this scenario possible, I have witnessed it first hand on our own range. It was was a few years ago during a patrol rifle certification, at a time when we would allow Officers to borrow another’s rifle if they were waiting on their own.
This particular rifle had a small piece of duct tape over one side of the hammer pin. Unknown to us, the pin had been walking out on the owner, so he decided to fix it with duct tape. It was such a small piece, none of us instructors really noticed it. After the fact, we would later diagnose that the hammer left the factory without a “J” spring installed.
At some point during the class, the tape came off and the hammer pin walked out far enough that it lost contact with one side of the receiver. Once that happened, the hammer itself fell out of alignment with the disconnector. We were up on the line, with me not standing too far behind said rifle. We gave the command to load up for the next course of fire, and the Officer puts one in the dirt about 2 feet in front of the line. I run up of course, call a cease fire, and tell him to make it safe. He tells me that it won’t go to safe, so he immediately trys to rack the round out so it will go to safe, (before I had a chance to tell him to freeze). As soon as he does this, another round launches into the dirt in front of him.
At this point, I’m close enough to snatch the rifle away from him, drop the mag and pulled/hel the CH to the rear. We step off line, and I’m bewildered that his finger had not been on the trigger when this occurred (at least the second instance that I could see). I double cleared the rifle away from the class, and break it down with a few other instructors.
We immediately see that the hammer is out of whack and pin had walked. On top of that, there was a few chunks of gravel rattling around in the FCG. All of it combined, caused the accidental discharge. It was a mechanical failure, through no fault of the Officer. Further inspection of the rifle found the missing “J” spring. The rifle was a Rock River, no suprise there.
We eventually inspected any Officer’s rifle who owned a Rock River, and found at least one other missing “J” spring. The event caused us to stop allowing the use of borrowed rifles on the range. When we contacted the owner, he acknowledged that the pin walked on a regular basis, and he never told the Officer who borrowed it. This incident, along with a few others involving Rock River, was enough ammo to take them off the authorized list. It also helped me get BCM approved, so I’d say it was a sweet trade-off. We also added a note to our class room portion, that duct tape is not an authorized method for repairing an AR.