What is the danger of placing a 20-gauge shell in a 12-gauge shotgun?

Placing a 20-gauge shell in a 12-gauge shotgun poses a significant danger because the 20-gauge shell is smaller in diameter than what the shotgun is designed for. If the 20-gauge shell is loaded, it may not completely exit the chamber upon firing, potentially getting lodged in the barrel. This obstruction can prevent subsequent shots from exiting properly, leading to increased pressure build-up when the shotgun is fired again. Consequently, this can result in a catastrophic failure of the shotgun, which may lead to serious personal injury to the shooter and potentially others nearby. Therefore, understanding that only the correct gauge shell should be used in a shotgun is crucial for safe handling and operation.