Yeah, I didn't know that a 16 gauge shotgun shell would chamber and fire in a 12 gauge Browning A5. It sounded funny and after checking we discovered the mistake. Too many boxes of shells on the table. No harm except for embarrassment at making a rookie mistake.
Good thing it wasn't a 20ga!...
You guessed it. When I put a rod down the barrel to check for a blockage, a 20 gauge fell back to the bolt face. I removed the shell, checked for any other obstructions, and then checked his shooting pouch. Three more 20 gauges in with the 12 gauges.
The bolt on the Benelli came forward with enough force to jam the 20 gauge round into the chamber; another shell into the chamber would have caused a catastrophe.
After the outing was done, we took an identical Benelli, trimmed a couple of empty hulls to correct length to see if both rounds would completely chamber. They would. This might not work on a 12 gauge with 2.75" chambers, but the Benelli would seat the round deep enough because the chamber was designed for 3.25" shells
Ok,now this part I don't quite understand. You cannot "jam" a 20ga shell in a 12ga chamber. Its too small. Normally what happens is the 20ga falls through the chamber and sticks in the 12ga barrel a few inches ahead of the chamber. Since the chamber is completely empty now (with the 20ga shell stuck in the bore) a 12ga shell can be chambered with no indication there is anything wrong. This leads to the infamous, and dangerous "12-20 burst", where the barel blows open at the obstructed point when fired.
Now, if the 12ga gun design allows the extractor to hold to 20ga shell in the chamber, its still grossly undersized and would certainly burst, if it went off at all. Not a good situation, either. This risk is why US ammo makers used (mostly) yellow/orange for 20ga shells since the 50s, and never (or rarely)used those colors for 12ga.
Not sure if this is still the case, but for many decades a yellow shell meant it didn't go in a a 12ga. Every other color could. When I was a kid, they taught that in Hunter Safety training. And, they had pictures! They also had some very ...sobering pictures of why its stupid to have loaded guns in cars. Ah, youth....
There are a number of rounds that will fit into a .357 mag chamber, including some that should not be fired. Bottom line is that if it fits in the .357 chamber and the cylinder closes and rotates normally, it won't turn your .357 into a grenade in your hand (factory ammo only, bad/wrong handloads are a completely different matter!!!!).