Knexfan won't stop bugging me about this so I'm posting this as a slideshow (just kidding...). All of this is copied directly from this page.

Dual M7 Caseless SMGs

I made another SMG and a Battle Rifle! The SMGs are almost exactly alike, except when I made the second one I noticed some problems and made a few small changes. Also, I didn't have enough of those swingy pieces I used for the forestock joint, so I used ball and joint pieces, which worked great.

Here is the description from the page I made when I made the first one.

This is a K'NEX model (just a model, not a gun) of the M7 Caseless Submachine Gun from Halo, better known as the SMG. It has twin notch sights, rear and fore stock, and a solid barrel. The stock is extended, the way I made it, but I'm thinking about making it collapsible. It should be fairly easy to put in a firing mechanism, but I don't plan on doing that right now. I don't think I'm gonna post instructions because I would have to take it apart. If anyone can dissect it in their minds, go ahead and make an Instructable, but make it a collaboration with me.

M7 Caseless SMG on Halopedia.org
High-Res M7 Caseless SMG

BR55HB SR - BR: Battle Rifle - HB: Heavy Barrel - SR: Sniper Rifle

Differences between BR55HB SR (Halo 3) and BR55 (Halo 2)
- Longer barrel
- Trigger guard

I based my model off the BR55HB SR, which is the actual name for the Battle Rifle in Halo 3, so it includes all the changes. It has the elevated rail, which has a scope on it. You might notice at the end of the barrel in the picture of the actual gun, the barrel looks expanded at the end. I did this too. The inside of the model is almost completely hollow, so it would be easy to convert into a gun. The barrel at the end would be hard though. The whole thing is very solid, even though it's hollow. It's also very easy to make, unlike the SMG.

BR55HB SR Battle Rifle on Halopedia.org
High-Res BR55HB SR Battle Rifle