What could happen if a 20 gauge shell is used in a 12 gauge barrel?

Using a 20 gauge shell in a 12 gauge barrel can lead to serious safety hazards, particularly because the smaller shell has the potential to get stuck in the larger barrel. When the 20 gauge shell lodges in the barrel, and a subsequent round of ammunition is fired, the pressure generated from the ignition can cause a catastrophic failure, resulting in an explosion. This poses a life-threatening risk to the shooter and anyone nearby, as it can damage the firearm and cause serious injuries.

The other scenarios, while plausible, do not accurately reflect the high-risk nature of this situation. The claim that the shell fits perfectly is incorrect, as the 20 gauge shell will not match the dimensions of a 12 gauge bore. Suggesting that the gun will not fire does not take into account the possibility of the gun being fired after the smaller shell becomes lodged. Also, the assertion that no damage will occur is false, as the potential for an explosion presents a significant danger when using improperly sized ammunition.