You may safely fire a 3-inch shotshell in a shotgun chambered for 2 3/4-inch shotshells.

Firing a 3-inch shotshell in a shotgun specifically chambered for 2 3/4-inch shotshells is unsafe and could cause serious malfunction or injury. The main reason is that the shotgun’s chamber is not designed to accommodate the longer shell, which can lead to issues such as obstruction or excessive pressure when the longer shell is fired. Shotgun chambers must match the shell length they are designed to fire to ensure safe operation.

In general, using ammunition that exceeds the specifications of the firearm can result in catastrophic failures, including barrel bulging or rupturing, which can endanger the shooter and those nearby. Therefore, it's vital to always adhere to the specifications provided by the shotgun manufacturer regarding the correct shell length that should be used. This practice is essential not only for safety but also for optimizing the performance of the firearm.