I found this on the MLG Forums. It works only if you have host, but could possibly work for everyone if playing on LAN. No confirmation yet. 
    
Here's the OP: http://www.mlgpro.com/forum/showthread.php?t=338119 

MLGpro.com b0rJa

Well guys, it's true... bxr is back it can be done consistently ONLY if you have host.
When i saw the bxr thread a while ago i turned my xbox on an started to mess around with the dmr, so I found out the button combo is RRYB, this would work sometimes, until i realized its more of an R (R+Y) B...

The timing has to be very precise.

the timing between the R and the R+Y has to be in between "spamming the dmr" and "timing your shot"
I really don't know if you guys will understand it but i don't know how to explain this 

Right after you R+Y, hit the melee button and voila! 

How it works? you fire a shot then fire another one quickly while switching weapons, then you shoot another DMR round having your secondary weapon out. So, basically you shoot once, then hit R+Y then melee and then the second shot that was saved releases. 

Against Someone

So I actually started to do this consistently while my friends couldn't (I was hosting), so I thought this wouldn't work off host, so a friend hosted a game and suddenly I couldn't do it and the host was finally able to do it. In my screen it looked as if the second shot happened but it didn't.

So there you go, jsut go and try it, maybe it can work off host, but i just couldn't do it, nor my friends.  

Can't wait for Bungie to patch it.  Why can't people just play the game and not try to exploit it?

I'm glad something like this has come out. The BXR and BXB made Halo 2 unpredictable, and some camper with a shotgun could get dominated by someone who could pull off a nice BXR. With the ability to use Active Camouflage whenever you like, a button combo is all it would need to shut them up. 
 
Bungie will patch it, however. Hopefully by then they'll also put out an MLG playlist so i don't have to mess around with the stupid armor abilities. 

" Can't wait for Bungie to patch it.  Why can't people just play the game and not try to exploit it? "

" Can't wait for Bungie to patch it.  Why can't people just play the game and not try to exploit it? "

 Nice find.  
 
PS: If a mod is reading this, I've been banned from creating topics for double posting for a quest. That was my first offence and have been banned for over a week. Can any one help me with this? Am I perma banned?