The standard-issue Assault Rifle for Hoplite soldiers, the Gladius AD57. The lower barrel serves as the assault rifle part, and the upper barrel can fire the larger, manually loaded, self-propelled exploding rounds. These larger rounds pierce before exploding. These rounds are carried mostly by the stronger, well-trained Praetorian Guard, or the elite of the elite, the Scholae Palatinae, who guard the Imperial Palace and who are bodyguards of the Emperor.
I too, am very impressed with the gun, and I like the flag also. Reminds me kinda of Warhammer. Check my page shortly because I built a sniper rifle coined off of your assault rifle design. You might like it.