The logical place to start is by looking at the two guns side by side. We went to the Stuffo weapons lab/firing range and put both firearms on the bench for a good hard look.
The M6D is a magazine-fed handgun that features a smart-linked 2.5X scope connected wirelessly to a soldier's HUD. A powerful and reliable sidearm, the M6D can be used in virtually all situations. It fires 12.7mm semi-armor piercing high-explosive rounds and has a 12 round magazine.
Let's face it; the pistol is bad with a capital "B." As I'm writing this paragraph my girlfriend is playing Halo:CE and I'm still mesmerized by the power and precision of that robust little hand cannon. No matter what, when playing Halo, I always have the pistol with me. In fact, I have been known to hold onto an empty pistol for quite a while. Even when it's dry you can still use it as a telescope.
The BR55 battle rifle is a medium range, light infantry rifle with an integrated 2X scope. It fires 9.5mm, armor piercing bullets in three shot bursts out of a 36 round magazine. It is ideally suited for assault roles. The battle rifle also serves as an effective long range weapon. What it lacks in power, it makes up for in accuracy.
In Halo 2 the battle rifle is hands down my favorite gun. It's not always appropriate in every situation but if all you have is the battle rifle -- you're still doing OK. Like many other people who have sent us their wish lists, I wish there was a way to switch between single shot and burst fire. That being said, it's still a heck of a rifle.