Here is the Ultimate Firearm. It has a drum mag for extended combat time, adjustable stock for versatility, an extended barrel for greater accuracy, a built-in grenade launcher with a grenade magazine, a built-in bi-pod for stability when sniping, and an optional tripod.
About this creation
I made an optional tripod for it and gave it like some kind of targeting system or something.
Bi-pod for sniping.
I gave it an adjustable stock. This is the stock fully extended.
This is the stock fully in.
A side view where you can see the grenade magazine.
Here's an approximately 200 piece bionicle modeling the gun in a Master Chief-ish pose.