Happened to me a few years ago while shooting a CZ 455. In my case the round didn't fire. I cycled the action and chambered the next round no problem. Fired and kaboom! I found the ejected squib had a neck split. and was still filled with powder that was packed in the case. My guess is the bullet from the squib stuck in rifling and pull out of case when I cycled bolt. Next round chambered behind it and pushed that bullet back into case. Magazine was blown out the bottom and into many pieces. Rifle was luckily not harmed. One small gouge in left forearm. I was shooting at 50 feet and found holes from both bullets on target 4-6" apart and out of round holes. I was shooting old ammo and found several more with neck cracks and splits.

 

I had this happen with a CZ452 - magazine destroyed, floorplate bent, stock split. Brass bits peppered my face (several blood spots on my cheeks, and around 4-5 chips in my sunglasses lenses. Bad day all in all. Hornady paid for everything, it was a known bad lot of ammo.

 

I'm in the habit of reaching my left hand over the scope and next to the chamber to grab the shell as it's ejected. This is just because I shoot off a porch and the shells drop through the openings between the boards. Then I later find myself scrounging around among the roots and firewood under the porch when I have to 'police the brass'. So, earlier this Summer, when the gun didn't go bang, and the shell came out empty, I knew I had better look. Sure enough, a round was stuck in the barrel. It ended my shooting that day as I didn't have a rod to drive that bullet out. I had to whack that thing too, it was really stuck.
Glad to hear you guys weren't hurt, that's scary......

 

For what it’s worth, I took my Winchester 1885 .17 hmr to the range with me a while back to fill in while my center fires cooled down. I took 2 different kinds of ammo, one was Remington 17 grain, and the other was Hornady 17 grain. I shot 15 rounds of each at 100 yards. The Remington 17 grain sucked in my gun and had like a 3” shotgun pattern. Just crappy ammo, or at least this gun doesn’t like it. Shot the Hornady, and all but 3 of them went into a nice 1 1/4” cluster. I saved the brass and brought it home and after looking at the targets I noticed the 3 Hornady’s had shot low. I inspected the brass and found 3 Hornady’s with split necks, which would explain the 3 low shots. Well today I was bored and got the Hornady out which was 2 of the 200 packs I got from Midway about 3 years ago. I looked at all the unfired rounds, and including the 3 that split a while back, out of 400 rounds I had 79 split necks. 76 are unfired.☹

 

Have you checked recalls? Someone else posted a split neck. That doesn't sound good does it?