Your easiest bet is to edit the native guns stats to fire/reload faster and in rounds.
If you want to go a step further and want a modern rapid-firing assault rifle, things get a bit more complicated. Easiest way would be to take a gun from some mod (Modern Warfare/Star Wars mod or something) and import it's model and mesh into a new .brf file using brf editor. Then you'd have to create a new item using item editor, apply the model and mesh from said mod to said item, and make it have the firing/reloading animations of the native musket. There's also a way to code the weapon to have automatic fire, but I'm short-knowledged on coding stuff, so yeah.
When you want to have different, unique sounds for the weapon, you'd have to do things on which my knowledge is short, and I'd have to try some things out myself before I'm really sure on how to add sounds for your rifle. And I'm just too lazy to try that right now.
Thing is, you don't just go around and add something as complicated as a modern rifle, especially when it's not a ready-packaged OSP file. I don't know a lot about modding M&B, but fully-functional rifles are something many modders have tried experimenting with, and only a few highly-talented modders managed to pull it off.