What could happen if a 20-gauge shell is placed in a 12-gauge shotgun?

Placing a 20-gauge shell in a 12-gauge shotgun can lead to serious issues because the 20-gauge shell is smaller in diameter than the 12-gauge chamber. If the 20-gauge shell is loaded, it may not eject properly, and can become lodged in the barrel. This blockage is highly dangerous, as firing the shotgun with an obstruction in the barrel can cause catastrophic failure. The pressure buildup from a fired round could result in the shotgun bursting, potentially causing severe injury to the shooter and others nearby. Safety protocols dictate that only the correct gauge of shell should be used in any given shotgun to prevent such life-threatening situations.