It is safe to fire a 20-gauge shotshell in a 12-gauge shotgun.

Firing a 20-gauge shotshell in a 12-gauge shotgun is not safe because the two gauges are incompatible in terms of size and pressure. A 20-gauge shell is smaller in diameter than a 12-gauge shell, and attempting to fire a 20-gauge shotshell in a 12-gauge shotgun could lead to several dangerous situations.

Firstly, a 20-gauge shell can become lodged in the barrel, which may result in a catastrophic failure if a subsequent 12-gauge shell is fired. This can cause serious injury to the shooter and bystanders, as well as damage to the firearm itself. Additionally, the differing pressures generated by each gauge can also lead to malfunctions or even explosion of the shotgun, as it is not designed to handle loads that are not specifically tailored to its gauge.

Thus, understanding gauge compatibility is essential for safe handling and operation of firearms. Always using the correct gauge shotshell for your shotgun is critical in maintaining safety when hunting or shooting.