When one thinks about Byzantine firearms one inevitably stumbles upon the famous "Greek fire" or "υγρόν πύρ", the ubiquitously mentioned used liquid used by the Byzantine from the mid-7th century. Its exact synthesis has been succcesfully kept as a secret throughout history though most modern historians tend to agree that it consisted of a mixture of sulphur, pine resin, naphtha and quicklime. It's liquid nature would allow it to be projected by highly pressurised siphons. Byzantines would take advantage of its abillity to burn even on the surface of water during naval battles, although images of hand-held siphons also exist, allowing for speculations of its use during sieges.
Despite the success of "Greek fire" in Byzantine history, the Empire was not able to take advantage of the introduction of gunpowder in military affairs. Coinciding with the complete degradation of the Byzantine military and political presence gunpowder firearms were hardly used by the Empire.
The first use of gunpowder artillery in the East was recorded in Hungary in 1354, in the Western Balkans in 1378 and in Serbia during hte 1380's. There is a questionable mention of Byzantine use of guns by John VII in 1390, against the fortress of the Golden Gate, held by John V. Issues of translation have allowed historians such as Bartusis to question the appearance of guns during this episode. According to Chalkokondyles, guns were used to defend Constantinople against the Ottomans in 1422. These guns were most probably obtained by the Venetians and Genoese since there is no evidence that the Byzantines manufactured any themselves.
Although the Ottoman guns played a significant role in undermining the Theodosian walls of Constantinople in 1453, the Byzantine themselves had some artillery that responded insufficiently against the Ottoman barrage. These included some large enough (firing 40 kg shot) that according to the eye-witnesses caused more damage to the walls than the enemy forces. Leonard of Chios mentions that at some point of the siege, Giustiniani asked the megas dux Loukas Notaras for the use of the public bombads of the city but was refused, indicating the presence of Byzantine guns during the siege.
Concerning hand-held firearms, there is no evidence of their use apart from a reference by the historian Doukas (writing in 1462) of Byzantines shooting lead balls, propelled by powder, 5 and 10 at a time as small as Pontic wallnuts.
The reasons for the lack of significant use of firearms in Byzantine history derive from the simlple fact of chronological coincidence of events: by the mid-14th century, the Empire was a sorry shadow of its glorious past, unable to keep up with new trends of technology due the lack of funds. Ironically, for an Empire that had always managed to adapt with the new political and fiscal realities of each period of its history, time and lack of funds did not allow Constantinople to fully utilise the lethal potential of gunpowder.