Here's the exact situation, with images:

The NEF Handi-Rifle is clearly marked .223 and the cartridges I bought were also .223 - not match-grade, just some generic .223 from Sportsmans guide:


I checked the barrel for obstructions, there were none.
I inserted the round into the chamber (this is a break-action gun), snapped it shut, made sure it was locked, cocked the weapon, got steady on my target, and pulled the trigger.
The blast wasn't right, and of course, there was that mist of brass on my right cheek.

The case ruptured quite completely:

Part of it fell right out, the rest needed to be pried out with pliers.

You can see pretty well where the firing pin SLAMMED into the cartridge - there's quite a deep impact there.



In looking over the gun back home. I can't tell if the break action lock was a factor or not - I noticed that once after I snapped it shut and checked that it was locked, the next time I handled the weapon, the action opened! Now I can't be sure if what I thought was a loaded, locked breech was in fact a problem. Although the gun was locked after the cartridge blew apart, I wonder if it somehow came unlocked during ignition of powder?

Anyway, that's about all the detail I can give, short of the weather at the time of the accident.