so you find yourself in 272 bc....

[Andy1984]

Since you have no instructors and your soldiers lack even the most basic knowlegde of gun-warfare, it would take decades to train your army. There is litterally no-one who has even a remote clue how to aim, fire and reload a gun. Let alone he'd do it when being charged by some gaesaetae. You'll need to have some accuracy and firepower to make volleys deadly. You'll still have to armour your soldiers (since your opponents will use missiles), which almost forces your soldiers to reload these guns when fully armoured. I really want to see that happen. You can no longer have any kind of decent shield. You'll have to equip your soldiers with something that is capable to break a charge of heavy cavalry...

To me, the introduction of guns seems at best possible in some very specific situations: you need to be able to train your units (no berserkers please) and to have a proper battlefield (no northern-european ambushes, no quick skirmishing raids,...). You'll need to have decent accuracy and enough manpower in order to make your volleys deadly (just imagine being encircled, being heavily outnumbered, or being utterly butchered in man-to-man fighting). You'll need to be capable to find ammunition, to keep your powder dry, to have decent supplies of powder and ammunition (swords can be repaired if needs be)... When you finally assembled your army you should avoid barbarians living in trees or opponents who rely on archers or any kind of heavy shock unit - whether infantry of cavalry.