I have had, and I saw one, AR15 slamfire in the standing stage of NRA Highpower competition. In that game you have to load single shot. Back in the day when the M1a ruled the firing line you could rest the rifle on your shooting stool and load it. Nothing bad happened because you rested the rifle on its butt on the shooting stool. Gravity slowed the bolt down, never heard of a slamfire. But, the AR15 came on the scene and I saw AR15 shooters putting the muzzle on the shooting stool, dropping a round in the chamber and dropping the bolt. I heard of, never saw, but someone's AR15 slamfired through their shooting stool, and the NRA changed the rule. No more loading a rifle on the shooting stool. My slamfire was with the muzzle pointed down, dropped the round in the chamber, hit the bolt release, bang! Same thing happened to my shooting bud who was scoring me. He got up there, dropped a round in the chamber, hit the bolt release, muzzle pointed down, bang!. I was using the new brass finish WSR, he was using Federal match primers. He quit using Federal Match after his second slamfire in the standing position. I went to CCI #41's.
After that, I would level out the rifle, pull the charging handle back, and lower the bolt half way, before letting go. I did the lowering the bolt thing in all positions except prone. Too difficult to do when laced up in a sling.
The early AR15's/M16's had a heavy firing pin and the things were slamfiring in service. The Army went to a lighter firing pin and a less sensitive primer. The things will still slamfire on occasion, less sensitive primer and all. The early M1 Garands must have slamfired, because the heavy, round Garand firing pin is a collector's item. I have only the picture, the things sold out for $100 each before I could get one.
That spring is a safety measure on AR10's. The rifle will slamfire without it.
When chambering a round, do not point an AR15, or any other semi automatic rifle with a free floating firing pin, at anything you don't want to put a hole in. I have accounts of in battery slamfires, and out of battery slamfires, with every action, except the roller bolts. One may have happened with a roller bolt, but that firing pin spring, I believe truck suspension coil springs are weaker. That firing pin is not moving forward to make contact with the primer, until the lugs are fully in engagement. This is a great design.