Never a good thing when a cartridge detonates within a firearm and the path of the projectile(s) is somehow blocked. That's what I surmise transpired during this incident.

I have witnessed 3 similar incidents where the shotgun had a massive come-apart, and the non-official conclusion was the reloads were lacking a wad. How the reloading process was manipulated to allow a bogus shell to pass "inspection" is beyond me? Possibly a double charge of shot/powder to fill the void? Either way, barring the possibility of a smaller gauge shell being inserted into the chamber, then a full-size shell backing it up, which would cause this kind of catastrophic failure. A live shell lacking a wad will allow the lead shot to liquefy, also allowing pressures to climb well beyond anything the guns are designed for. The end result is as you witnessed.

All 3 incidents I witnessed were people shooting reloads they had purchased from someone else. One little fella had the most beautiful Belgium Browning Diana Grade O/U. He'd just purchased it NIB, yet it was over 20 years old. He drove a new Mercedes, and he was at a registered competition. Yet he found some local-yocal to purchase reloads from at the shoot!!! We were standing back under the shade trees, just hanging out, and KA-BOOM! I happened to be looking "at him", but not watching him shooting. Upon hearing/seeing the incident I watched in s-u-p-e-r s-l-o-w m-o-t-i-o-n the forearm explode in 3 pieces and he got the [Bleeeeep!] kicked out of him!! He then slowly turned around (gun still shouldered) and was close to pointing it at the crowd of spectators, as he was honestly in shock, and another shooter came out of nowhere and knocked him to the ground!! The event was 16yd singles, so he only should of had one shell in the gun (which he did-but this dude seemed like a pretty new shooter, was hard to understand with his thick accent, and overall was just kinda goofy..as I saw him several times that summer).

I reload on a couple of Ponsness/Warrens and you can get a double charge of shot/powder. If you do, you have a double charge of each though, and the manual spells this out MULTIPLE TIMES! Shot spills all over the table, and that shell is phuckin' useless. Now the shell with the double charge of powder..I guess if you're an absolute IDIOT, you could forget to put a wad in it and then add shot and see what it looks like?? IDK, (knocking on wood)..I've loaded A BUNCH, and screwed up plenty, and haven't gone there yet.

I don't recognize the gun in the picture...trigger looks kinda different too? Were the shells reloads? Congrats on your first 25! I remember mine, 1986. Borrowed BT-99, Federal Paper reloads, reclaimed shot, 18.5 grains Red Dot, Fed Primer, Win AA12 Reclaimed Wads (yes, I picked up wads & recycled them). 4th-week shooting trap, man I was hooked!