What is one of the notable faults of the RPD?

The notable fault of the RPD, specifically regarding its difficulty in changing barrels, is significant because quick barrel changes are crucial in sustained fire scenarios to prevent overheating. In automatic weapons like the RPD, the ability to replace a barrel quickly allows operators to maintain a high rate of fire without risking weapon failure due to overheating. The RPD's design generally requires more time and effort to change barrels compared to other machine guns that are engineered with more user-friendly mechanisms for quick barrel swaps. This characteristic can hinder operational effectiveness, especially in combat situations where time and efficiency are critical.

The other options highlight characteristics of the RPD that do not relate directly to inherent faults or limitations in performance. For example, its lightweight design is typically regarded as an advantage in terms of portability, while the ability to disassemble quickly is beneficial for maintenance. The feature of automatic fire is standard in many machine guns, so it does not represent a fault either. Therefore, the difficulty in making barrel changes stands out as a notable drawback of the RPD.