Generally, gunblades exsisted during the early days of firearms, when the weapons were single-shot, impossible to reload quickly, generally inaccurate, and prone to failure. They were meant to give the user a fallback weapon incase he missed or the gun misfired. the only problem was that the additional weight meant the designs either had a flimsy gun mechanism, or a heavy, off-balanced sword. (the main reason why they were not very common and fell into disuse)

Though any bayonetted rifle would essentially be a gunhalberd.