An assault rifle that is illegal and pumps out more rounds per minute than any Imperial carbine or rifle. The shots are disruptor type, meaning they destroy things at a molecular level and they ignore all personal shields.
In short, imagine slagging someone to death with minifireballs or something. Really gruesome weapon.
I like Mandalorians in warfare.
Haven't had the time to work on your sig yet, and Boba and Jango obviously don't need such heavy weapons, since they aren't in war and they never attack fortified positions, large groups, nor more than perhaps one or two jedi opponents. Notice that when Jango got into a situation where there was more than zero jedi on the planet he fled. They are bounty hunters, and as such they pick their battles.