We analyze the practical limitations of a secret-key generation system from channel gain variations in a narrowband wireless environment. We examine the different assumptions usually made for theoretical purposes and experimentally verify to which extent they remain valid in an actual system based on software-defined radios. We not only characterize the source of common randomness induced from channel gain variations, but we also estimate achievable key rates in the finite key-length regime. We show that a secret-key generation system based on channel gain variations is extremely sensitive to external modifications of the environment, and that the system should adapt accordingly to guarantee a given level of information-theoretic secrecy.