For which condition should black powder and smokeless powder never be interchanged?

The correct choice highlights the critical distinction in the use of black powder and smokeless powder, particularly in rifles specifically designed for their use. Rifles that are engineered to handle smokeless powder typically have stronger materials and designs to withstand the higher pressures generated by this modern propellant. Black powder operates at significantly lower pressures and has a different burning characteristic, which can lead to dangerous situations if mistakenly interchanged.

Using black powder in a rifle intended for smokeless powder can result in catastrophic failure, including damage to the firearm and serious injury to the shooter. Conversely, using smokeless powder in a muzzleloader (where black powder is standard) can also lead to dangerous circumstances, but the focus of the question is on the specific designs of rifles tailored for each type of powder.

Understanding the distinction between these two types of powder is crucial for safe firearm handling and operation, emphasizing the importance of using the correct type of powder for the specific design of the firearm to prevent accidents and ensure safe shooting practices.