I have never seen a true firing pin initiated slamfire in a AR when feeding from the magazine. I have had one, and seen one, and heard reports of others from folks who single fed rounds and had a slamfire.
And it makes a believer out of you, regardless of what is said by those who say it can’t happen.
That does not mean that you did not have an overly sensitive primer. One that ignited when the free floating firing pin hit it.
Still for gas guns, I highly recommend sizing the cases with a small base die (so there is no resistance to bolt closure), seating all primers by hand, and using the least sensitive primers you can find.
And there is one other thing, don’t load these rifles pointing at something you don’t want dead. If you have a slamfire, you just killed what was downrange.